How many social networking sites do you use?

Daily I look at Facebook, Twitter, Identi.ca, LinkedIn, Yammer, and probably a few more I use far less often.  But what do you do when a friend or colleague invites you to use YASNS (yet another social networking site)?

I’ve been sitting on an invite to Namyz for ages.  It was sent by a friend of mine, and I don’t want to snuff his enthusiasm for this particular site, but I don’t really want yet another of these to keep track of.  Same goes with Plaxo.  I’m sure they’re wonderful, but really, how many can one person be expected to use?  What’s the etiquette for saying “no thanks to Namyz, but if you’d care to send me a LinkedIn invite, I’d accept that?”