Is this already happening? Fatigue after the initial euphoria over Facebook applications.
Last I checked, there were a total of 1,729 applications on the Facebook Platform. Majority of them, over 95% of the total, had less than 100k users. Only 24 applications had over 1M users (~8.5M for the most popular app). This highly lop-sided usage, though logical to some extent, still needs an explanation.
Given that Facebook's ~30M users are very active (50% visit the site every day), it may appear that adoption rate is stagnating within six weeks after Facebook's f8 Platform launch on May 24th. I'd argue, at the risk of being blamed for premature judgement, that many developers are throwing in crappy applications without giving too much thought into what would work on Facebook. "If you build it, they'll use it" does not apply. Fundamentally, such applications in a community use the real connections users have to help them share information more efficiently. Applications not designed to leverage the power of those real connections (Facebook calls it social graph) will not be widely adopted. And if adopted, those minority users won't use them frequently over time, or remove them altogether at some point. I've seen number of users on some applications actually go down with time, as users clear the clutter of applications they had added. With credit to Facebook, removing an application is as easy as adding one.
It'd be interesting to see some usage metric - e.g, number of invites sent/received thru an Invite application - in addition to the metric currently used to determine an application's popularity (number of users who added it to their profile).