Pinterest has finally confirmed co-founder Paul Sciarra (pictured, left) has left the company.

He’ll be serving as entrepreneur-in-residence at Andreessen Horowitz, one of the more important venture capital firms in Silicon Valley.

“Of course, I’ll continue to be there for the company: now, as an advisor, an owner, and — as always — a dedicated pinner,” Sciarra writes on the company blog.

Yesterday, the rumors ran rampant that Sciarra was leaving Pinterest behind. Given the startup’s recent and rather meteoric climb to web traffic dominance and mainstream success, we weren’t the only ones who found Sciarra’s timing troubling.

“Although it’s hard to imagine today, I lost sleep when we launched the first version in early 2010 over whether anyone except a handful of close family and guilt-ridden friends would ever use it,” Sciarra noted in the post. “Now, millions of people from all over the world use Pinterest, and it’s on a trajectory to make pinners of so many more!”

In his new EIR role at Andreessen, Sciarra will be advising one of the most exciting portfolios of startups the tech community has to offer. Andreessen recently raised a $1.5 billion fund to continue its investments in startups like Airbnb, Zynga, Bump, camera maker Lytro, Skype, Twitter, and yes, Pinterest.

“I hope that eventually leads me back to doing what I’ve done for the past three and a half years — namely, building cool things with awesome people with unexpected results,” Sciarra concludes.

Sciarra and college pal Ben Silbermann launched Pinterest in 2010. Today, the startup could be worth as much as $500 million. Members use the virtual pinboard site to pin products to collections called boards, an activity that’s become so popular even the president is doing it.