Questions about social networking--useful tool or pointless time suck?

I'm going to an old-skool networking event tonight--you know, where you meet people face to face.

Tonight's theme is social networking. And the woman in charge has asked me to attend because I'm the biggest geek she knows.

Now, I know what I use social networking for: I blog to share my high-quality blather with the world; I started the Mamarazzi Ning site because Mamarazzi devotees deserved more opportunities to bash celebrity parents; I tweet because I never shut up; I'm on Facebook to play with lil Green Patch; I'm on Good Reads because I supposedly care what my friends are reading, (except actually I don't because my friends read boring shit) and LibraryThing because I'm always on the look out for fellow Georgette Heyer and P. G. Wodehouse obsessed weirdos. I started a YouTube account to collect my favorites in one place, but now find it useful for spying on my children.

And I'm on LinkedIn because I write for BlogHer and felt a certain pressure to appear professional.

So this is where the questions (and over-the-top shameless comment whoring) come in.

Which social network sites do you belong to? Why did you join? What do you really find useful/fun? And is any of this practical, or is it mostly a big time suck?

Please leave comments--I'd love to be able to demonstrate the power of social networking to my real-life networking group!