I am also quite happy with the possibility of finding long-lost friends (although so far I haven't found any), family abroad (I did hook up with a handful of relatives on the other side of the globe), and getting to know the friends of friends.
But networks are strange things. Where I live, there are four main networks, and they only touch each other marginally. The four are: the native Luxemburgers, people who work in foreign banks, people who work for the institutions, and the Portuguese immigrants who came in the sixties and seventies. I thought, when I joined Facebook, that I would some magically tap into a vast reservoir of people and cross the boundaries of existing networks. In fact, the opposite has occurred. My collection of Facebook friends (as oppposed to family) is not only restricted to people I know professionally, it is restricted to a very small subset of people I know from or through work. And the subset consists primarily of Hungarians, Maltese and Greek friends.
When I came to Luxembourg, I remember feeling very much at home with all the other people that were in greater or lesser degree rootless. It was a relief after many years of feeling out of place. But now, thanks to Facebook, I am out of place in my own social network: Hungarian, Maltese or Greek are so far out of my reach I can only throw up my hands and say Oi Vey!