Earlier today a member of the XMPP developer community noticed that he could log into Facebook chat using a standard XMPP client. The news has since been confirmed by Facebook. This is great news for XMPP, and great news for open standards in general.
Although Facebook has not yet enabled federation with the rest of the public XMPP network, I'd imagine it is only a matter of time until that happens (after all, Google Talk launched in August 2005 but didn't federate until early 2006).
In any case, it seems that we'll need to seriously increase our informal estimates about the number of XMPP users. By about 400 million. :) --stpeter