Facebook doesn't have to join up. It'll take a half day's work to make a wrapper you can throw on your OpenSocial app to make it work in Facebook. If that.<p>It's in Facebook's interest to keep people making apps that have some attachment to Facebook proper, though. If people aren't using FBML and FQL, there's no platform lockin. I assume Facebook's going to beef up their custom offerings so that people are at least forced to make a special version of their product just for Facebook.