Kevin Marks, a social force within Google and one of the main drivers behind its recent social efforts (including OpenSocial, its Social Graph API, and Microformats) is leaving the company. He announced his departure today on his blog. Marks was an evangelist to other engineers outside Google, his official title was Developer Advocate. For many Web developers outside Google, he has been the public face of its social efforts in recent years.

Contacted by phone, Marks says he is working on a bunch of things “related to the social Web” and “activity streams,” but declined to get into specifics. Asked why he is leaving Google, he responds that his work is pretty much completed: “Over the last two years, we have built out the infrastructure for the social Web. Now it is time to build things on that infrastructure.”

He also is ready to work in a smaller company. Noting that he has worked at both large and small companies over the years, he says, “I am due for a small company phase.”

That is a nice way to put it. OpenSocial has been gaining steam, but still seems to be lagging Facebook as the platform of choice for social applications and Websites. That is not to say that the game is over or that OpenSocial cannot prevail over time (precisely by being more open). The action, anyway, is moving to real time activity streams and Marks now seems to be pointed in that direction.