Medium, the publishing platform started by one of Twitter's cofounders 3 years ago, has 25 million users and is worth $400 million

Medium, the blogging platform founded by Twitter cofounder Ev Williams, just raised a $57 million round, led by Andreessen Horowitz with "significant participation" from Google Ventures and Greylock Partners, the company announced in a blog post Monday.

Sources tell Recode that the new funding values Medium at $400 million.

Previously, Medium had raised $25 million in a round led by Greylock Partners.

Andy Doyle, Medium's head of operations, says in the company's blog post that 20,000 people are using Medium weekly. Doyle told Recode that that number is 4 times higher than last year. Doyle's post doesn't discuss traffic, or other metrics that may give a sense of Medium's health as a publisher. We do know, though, that Medium said it received 25 million monthly unique visitors as of May.

"At Medium we operate in a fundamentally different way," Doyle says in his blog post. "We don't focus on pageviews, unique visitors, or click metrics. We don't litter the platform with ads that are low quality, high clutter."

In terms of revenue, Medium hasn't disclosed much. In an interview with John Battelle earlier this month, Williams said Medium was starting to look at explore options to generate revenue, such as paywalls and branded content.

This summer, Medium - which has become known for tech and culture blogs like Matter, Backchannel, The Nib, and The Message - announced a change in its direction, moving away from its focus on longform content.

Structural shifts inside Medium were rampant, Business Insider learned earlier this year. This applied not only in editorial practice but in management as well. Medium told BI this the changes weren't considered a pivot or a shakeup in any sense of the word; it was just the evolution of the three-year-old blogging project.

When asked about the changes, Edward Lichty Medium's Head of Corporate Development and Strategy wrote to Business Insider: "All of our content team, including people working on Matter and other publications, will be part of the effort to help drive conversation on Medium. We don't know exactly what this will end up looking like, but certainly you'll see a broader variety of activity ... Our content team will continue to do great work like they always have, including in our publications."