Caspar posted a great article about the concept of Facebook Fatigue the other day. The main idea is that the initial buzz around Facebook was to see who had recently joined and get back in touch. Once you’re back in touch, however, the reason to go back fades.
This is probably why they’ve opened up their develop platform in order to provide new reasons to return – new applications.
I have always been of the opinion however that sites like these are at their best when they complement face to face relationships rather than replace them. So if people use Facebook a little less but get more real world value out of the time the spend on it, then that is a good thing!