In a post on Pinterest’s blog, Sciarra writes that he’s been hired as “Entrepreneur-in-Residence” at VC firm Andreessen Horowitz, an investor in Pinterest. With his departure, cofounder Ben Silbermann officially becomes CEO.

Content-sharing invitation-only site Pinterest launched in March 2010 and has grown remarkably fast in recent months, hitting 17.8 million users in February according to comScore. The majority of the site’s users are women aged 25 to 44, many from the South.

In his goodbye post, Sciarra touched on why he thinks Pinterest has been such a success:

[The] best internet services aren’t just ways for people to escape their everyday lives. Instead, those services with longevity — with real “legs — enhance folks’ day-to-day experiences, deepen their relationships, and show them things about themselves they didn’t know before.