SOURCES: Millennial Media was forced to sell to AOL after all its top managers left

Millennial Media sold itself to AOL for a fraction of its former value largely because all its senior managers had left the company, sources tell Business Insider.

At one point, a few years ago, Millennial was regarded as the vanguard of the nascent mobile advertising business. It staged one of the first mobile ad tech IPOs of the post-2008 tech boom, proving that there was enough revenue in mobile ads to sustain a public stock offering.

But as the stock tanked after its 2012 stock float, and then its revenues sank too Q2 2015, the receding tide revealed a company whose captains had long ago abandoned ship.

We left several messages with the company for comment but did not hear back.

There are lots of reasons AOL was able to acquire - or "mercy kill," as one writer put it - a company that was once worth $2 billion for a tiny fraction of that, just $238 million. But extreme management churn was a big part of it, sources say.

This is a list of senior managers who have left Millennial since it went public in 2012:

• Founder/CEO Paul Palmieri left in January 2014 after taking about $8 million in compensation over the prior three years.
• Founder/CTO Chris Brandenburg left in 2013.
• CFO Michael Avon left in 2014 to join ABS Capital.
• George Bell left in 2013. He was the experienced former CEO of JumpTap, one of Millennial's acquisitions.
• Mollie Spilman, evp/global sales and operations, left in 2014.
• svp marketing Mack McKelvey left in 2012.

In short, the founding team disappeared after the IPO. The top leaders responsible for the strategy, tech, sales, and marketing that got the company to the stage where it could go public simply aren't there anymore. The people running the show now are more recent arrivals, such as COO Ernie Cormier. He was the former CEO of Nexage, another Millennial post-IPO acquisition.

"It's baffling," a source tells Business Insider. "The founder/CTO & COO left in Q1 the next year [after the IPO] I believe and nearly the entire tech team went shortly after that. Then Paul [Palmieri] left early the following year. The CFO left about a year ago. None of the original management team remained as of last summer. ... it became a 100% different company."

The new CEO, Michael Barrett, joined the company in 2014. He would receive a financial reward if he sold the company, according to this SEC disclosure from 2014. If the company were sold, Barret would receive:

• An additional 12 months' salary ($476,000)
• 100% of his restricted stock units
• 100% of his stock options
• That package was worth about $10 million on paper in 2014, according to the disclosure.

Like all tech stocks, Millennial got its valuation on the promise of future growth. When it went public in 2012 its revenues were growing at 117% a year. It sat in front of two massive waves of money: The shift of ad dollars from old media into digital media, and the shift of yet more ad dollars from desktop web media into mobile apps. The sky was the limit, and its stock popped on its first day, rising from $13 to $25. Its market cap was larger than the entire mobile ad market at the time.

But tech companies can only sustain high valuations as long as it looks like there is more growth to come. When Millennial reported a decline in revenues in Q2, there was nowhere for the stock to go but down.

Put those factors together - an exodus of the company's leadership, declining revenues, stock going nowhere, and a CEO who would not suffer financially if the company was acquired, and you can see why this deal happened.

"I think the deal was inevitable, and I'm surprised it took so long," another source says. AOL is "weak in mobile" this source says, and MM at least gives it an established market to build upon.