A commenter ("Gethin") pointed out after a recent post that research had found a strong correlation between high friend counts on Facebook and frequently changing profile pictures and narcissism.
If you Google the topic, you'll find a number of such studies, including this one cited in The Guardian. It all makes sense. Having a high number of "friends" indicates a certain superficiality and a desire for more of an audience. And changing one's picture frequently betrays a certain vanity.
Something to keep in mind as you appraise your Facebook friends.
I have something like 30 friends on Facebook, many of whom I barely know, I've used only one profile picture (the same one I've used on this blog), and I've never put up a single status update. On the other hand, this blog is basically my Facebook-substitute, and I put far more effort into it than most people put into their Facebook pages. Not sure where that leaves me.