I have a Xing account... which I need to update. For international tech recruiting it is great for locating Germans.
Plaxo: Yes but have not found much use for it.
Doostang: Tried it a year and a half ago... looked to be good for finding Ivy League finance guys, but most ppl I found were also on LinkedIn.
I actively use recruitingingblogs, LinkedIn, and Facebook (thought this one is purely for social) recently opened a myspace account but have done nothing with it and have no friends.
i have been invited to more sites than i can imagine. the truth is with web 2.0 everyone with every type of interest can have a social networking group. since it is all user generated content, it is really excited. those that don't find an audience will go by the wayside.
you can be selective or you can join everyone, it really doesn't matter. if you don't use it no big deal, it won't be around too much longer anyway.