Google and Facebook are fierce rivals, and spar over advertising, access to private information, and talented engineers. Given the two companies' obvious mutual obsession, why is Google co-founder Sergey Brin so secretive about having a Facebook account?

Though there are pages and another personal account with his name on them, Brin appears to be using an account named "Sergey Sergey." The account carries no picture, but lists Google and Brin's alma mater Stanford University among its networks and Google's Chrome web browser as its lone public "interest." Well connected members of the Silicon Valley tech community have decided the account must belong to Brin. Though Brin keeps his friend list private and does not allow himself to show up other people's friend lists, people in his greater social circle are able to see friends in common and report that they are mostly, if not entirely, Google veterans, people such as Facebook chief technology officer Bret Taylor, a former Google product manager, and Chris Sacca, a tech investor who used to be Google's head of special initiatives.

Clearly, Facebook knows about Brin's account. So why doesn't he want anyone else to know? One possibility: Brin was recently made the public point man for Google's social initiatives, i.e. its war against Facebook. If he's going to start promoting some of Google's many social initiatives—Google Me, "+1," Buzz, Friend Connect, Google Profiles, Orkut—he might not want prospective users to know how deeply enmeshed he is in a rival social network.

If you've got a better idea of what's going on we'd love to hear it.