Why are all the apps on Facebook trivial, crappy or spammy?
Facebook originally launched the app platform with a lot of talk about how to create value out of the social graph.
Why hasn't that happened, with all the energy being spent on developing Facebook apps? Ben Rattray has a good answer - mainly because the structural incentives are all on creating virality.
The reason there are few and little use of utility-based applications is not because users don't want to use them or because app developers don't want to develop them, or even because Facebook doesn't want to encourage them (which they clearly do). It's because the means of distribution inside Facebook are structurally biased against them.
[I]t is very difficult to achieve a viral coefficient of over 1 through word of mouth. Ironically, this difficulty is compounded inside Facebook because the proliferation of viral action apps inundates users with invitations and makes them less and less likely to accept anything – including invitations to utility-based applications. So the barrier for going viral increases even further
Ben also categorizes the kinds of apps that could be useful, and which ones make sense on a social platform. It's very useful to look at - if you are thinking of how the web will be shaped by increasing adoption of "social" techniques into websites.
Of course there are more types of apps, and the list of examples is thin, but it's useful to think about if you plan to adapt to the ever-increasing "socialness" of the web.