Once they launched to the world at large, many people enjoyed connecting with people that had been in their lives previously like some big high school reunion. This is still the first thing that attracts a new user to fb.
The problem I see is, without a constant need to stay in touch (i.e. who's doing what, who's going where this week), it really has no purpose. Once I've seen what my buddy in grade 2 is up to now, and who my first girlfriend married, I don't really need to know anything else.
Hence: Applications. FB apps are the solution looking for a problem. In many cases, the app itself is what people like to use. For instance, scrabulous is pretty popular with my wife. The problem is that for someone like me, all the apps do is annoy the hell out of me, and make me less likely to use fb.
I've seen more than a few friends suddenly drop off fb. I suspect the trend to continue.
If they're somebody I knew from my past, they don't. Because outside of a two minute catch-up when we first discovered each other on fb, we haven't said anything to each other anyway.
Why do you think it solves some problem?
My line of thinking was more from the point of view of "fixing things that are broken" as a way to create interesting products.
For example, Youtube was fixing the lack of an easy way to share video online.
That's exactly what it does
seriously.