So Facebook doesn't need to start worrying yet ...

OpenSocial People does not support queries.

If I read it right (after 5 mins, I admit) the API supports getting one friend by id or all friends. But no query that filters friends by link-type. Which, if you think about it, means that there really aren't any social applications that you can build which don't need to go beyond the scope of the API ... which means that all the interesting social applications are still gonna have to be platform specific or modified on a YASN by YASN basis.