The Facebook API may be great in many ways, but for stability there's still a lot to be desired. There seems to be a slightly "gung ho" attitude to code deployment at Facebook which, although I feel some empathy with, isn't particularly suited to a site with 50 million users. Take this recent developer update as an example:

Requests inadvertantly deleted Dec 11, 2007 10:27am It looks like all outstanding requests/invites from platform applications may have been inadvertently deleted last night. We are exploring various recovery options and will post here when we have more information.

So the last few days weren't a great time to release an application, it seems, and that explains why growth (and ad revenues) have been down on my half a dozen or so.