Here's who insiders think could replace Twitter CEO Dick Costolo

This story originally posted June 2. Today, Twitter announced that Costolo would resign effective June 30, with Jack Dorsey taking over as interim CEO.

Wall Street is not in love with Twitter CEO Dick Costolo, but finding a substitute won't be easy.

Twitter's mediocre financial performance last quarter, its ongoing challenges with stalling user growth, and Costolo's past stock sales have led many investors to wonder if it's time for a leadership change.

Those calls could become louder when Twitter has its annual shareholder meeting in San Francisco on Wednesday.

But there's one big problem for those hoping for a changing of the guard: There aren't any obvious candidates within the company who could quickly step in and turn things around.

Some observers point to head of revenue Adam Bain as the best suited to take the reins in the event of a change, though he's not a perfect shoo-in.

A string of personnel changes and Costolo's own particular style of promoting employees has left the company without a clear number 2 with all the qualities considered essential to lead the social media company, say some investors and former employees. While finance chief Anthony Noto has emerged as Costolo's right-hand-man, many question whether the ex-banker would make a good fit to lead an Internet company, particularly one with Twitter's challenges.

"I don't think there is an heir apparent internally," said one Wall Street analyst who covers the company closely, speaking anonymously because he was not authorized to speak to the press.

The situation may give Costolo some breathing room as he looks to revitalize Twitter's growth with a slew of new initiatives, including a recently-launched partnership with Google that will showcase tweets within Google's search page.

But the clock is ticking.

How long does he have?

"He probably has until the end of this year to show that he can deliver," said Walter Price, a senior portfolio manager at Allianz Global Investors, whose firm owns Twitter shares.

Price said that he's encouraged by some of the CEO's recent initiatives, such as a feature that will show users the most important tweets they missed while they weren't using the service.

But he added that for Costolo "to win my confidence he's going to have to show me the results," such as increases in number of users and in user engagement with the service.

Price met with Twitter management after the company's recent disappointing quarterly earnings report. The company's management appeared very focused on the problems at hand, he said. "They know what they need to do. I don't think they're clueless."

Of Costolo, Price said, "I think he's got the right idea of things they should be working on." He continued, "I guess my criticism has been his vision is too grand and they need to parse it into pieces that get them significant increases in audience and advertisers without executing to the grand vision in one step. Maybe make it in several steps."

In an on-stage interview at the Code conference last week, Costolo said he was not worried about losing his job.

"The board and I are totally in sync about what we need to do - we have a very clear strategy that we've articulated and that we're following," he said.

Twitter declined to comment on this story.

Fixing the product

Twitter is a popular communications medium for everyone from journalists and celebrities to activists and politicians. But the service can be challenging for mainstream Internet users to figure out, and Twitter has struggled to attract new users.

With 302 million monthly users, Twitter's audience is much smaller than Facebook's 1.44 billion users. Meanwhile, fast-growing upstarts like Snapchat, which now counts 100 million daily users, are catching up.

After missing Wall Street's Q1 revenue targets and warning investors that user growth in the current quarter was off to a slow start, Twitter stock plummeted 30 percent and currently trades between $36 and $37 a share.

Many observers and people close to the company believe the only way to jump start growth is to make the product more appealing. "When you get the product down, eyeballs will follow," said the Wall Street analyst.

If the company makes a CEO change, the new boss has to be a true product visionary, he insists.

But Twitter's product team isn't what it once was. A series of shake-ups over the past several years has resulted in many of its most respected product gurus leaving the nest.

The current head of product, Kevin Weil, began at Twitter overseeing analytics, and then moved to the advertising side of product initiatives before ascending to his current role.

Weil's career at Twitter is emblematic of Costolo's management style, said one former employee. If Dick trusts a person, he gives them a lot of opportunities to take on new challenges, the former employee explained. The approach can bring fresh thinking and energy. It's worth noting that since Weil took over, the company appears to have accelerated its pace of new product development.

Twitter CFO Anthony Noto is also taking on new, unconventional responsibilities. In May, Noto assumed control of consumer marketing at Twitter.

The former Goldman Sachs investment banker and ex-Army Ranger arrived at Twitter in July 2014 with strong credibility on Wall Street. But the shine is already starting to wear off.

"He put on a great analyst day, but we're back where we started," said the analyst, referring to Twitter's November 2014 analyst day in which Noto dazzled investors with bullish growth projections.

And while Noto may fit into the Internet company in the finance role, some people close to the organization don't see him as a smart choice for the top job.

"There could not be a guy who is less the spirit of Twitter than Noto," said the former employee, noting Noto's well-publicized problems using the product - on a couple of occasions Noto seems to have mistakenly sent private messages as public tweets - and his lavish pay package. Noto's $73 million total compensation of cash and stock in 2014 sticks out at a company where salaries were until recently capped at $200,000.

"It used to be that the highest salary was $200,000. Nobody made more than that and a lot of people made a lot less," the former Twitter employee said. Today, he said, "a lot of people are being paid a s--- ton of money and they have not been able to move the needle."

What about Adam Bain?

One potential wildcard is President of Global Revenue Adam Bain. The former News Corp executive, who joined Twitter in 2010, is credited with building Twitter's advertising business.

It's not clear that Bain has the product chops to be CEO - much of his work has been operational, such as building out Twitter's sales team.

But Bain has also overseen the creation of customized products and technology for advertising customers, according to Bloomberg. And his fans point to efforts such as Twitter Amplify, which incorporates sports video clips within Twitter ads, as evidence of a strong product vision.

That could make Bain a more popular CEO option within the Twitter ranks than some of his peers. While appointing a finance person like Noto to the top job might cause some employees to revolt or jump ship, in Bain's case many might say "he is a sales guy, but he has shown clever product insight," speculates one person with knowledge of the matter.

Bain has also proven adept at translating Twitter's vision to the public, a crucial skill at the company's current juncture. Twitter's November analyst day showed that Bain "is the most effective communicator at the company," said another person familiar with the matter.

But, this person said, Bain is being "actively being prevented from speaking on the earnings calls or appearing on stage in front of the public." Despite what the person said have been numerous requests from analysts and investors for Bain to speak during quarterly earnings calls, Twitter has not included him among the executives on the call. But Bain has appeared at industry events such as a March Re/code conference.

Outside saviors

Some believe the company's best bet would be a savior from the outside.

"I would think they would want someone from the outside because it doesn't look like they're getting anything right on the inside," said one portfolio manager, speaking anonymously because he was not authorized to speak to the press.

At the top of the portfolio manager's wish list for a new Twitter CEO: Instagram co-founder Kevin Systrom or Yahoo CEO Marissa Mayer.

Twitter co-founders Biz Stone, Evan Williams, or Jack Dorsey could also be good potential candidates, he said, though all three are currently engaged in other projects.

Costolo can probably hang on for a while, the person said. But if the stock falls further and hits the $20 level, the pressure for a CEO change may be insurmountable, he said.

Given Wall Street's current sour mood towards the company, Twitter needs to act fast, said this manager.

"People just want to crap on Twitter right now is what it feels like," the portfolio manager said. "It's like the fun thing to do."