Social network fatigue

I'm sorry, Yahoo, I really can't be bother with yet another social network, and I'm not the only one.

At present, there are three social networks which I use on a day-to-day basis: Plaxo (which isn't really a social network, as I use it to sync my contacts with everything); LinkedIn, which connects mostly business contacts; and Facebook, because it's good for meeting people that I haven't seen for ages.

What's interesting about Mash is that it's the first social network that I've seen launched where no one I know has posted anything to friends asking for an invite. Does this indicate that people are just getting bored of social networks - or is it that Facebook is doing a "good enough" service for virtually everyone?