The whole thing's daft. I'm always getting these emails saying so-and-so has just joined Facebook and wants to be your friend or whatever. Sometimes it's Facebook, sometimes it's another site. For example an email saying so-and-so wants to add you to their birthday calendar.
It's always someone who has me in their address book, but typically hasn't emailed in 2 years!
I never bother registering with whichever site is flavour of the day, but I do send an email to that person if I want to keep in touch.
I don't have time to spend all day talking to people on websites, I'd rather spend time and have friends in the real world.