Im on it quite a bit for a couple of reasons... I'm part of a team of people that organise and run a historic larp and, though we have a site and forum, not everyone is a member and/or dont check it very often, so facebook is excellent for keeping players informed.
Secondly, some of my friends are those type to have a billion email adresses and add new ones daily so guessing which account to use us impossible.
Third, and lastly, when I work nights I often pass the time by playing the games, like poker for example, which some of my friends also use.
I do think, as has been said, its only still ao popular because nothing of the same style has come along to replace it yet, like facebook did with myspace and bebo.
Mads