I was invited to LinkedIn by Atul on Saturday. I generally refrain from signing up on networking sites unless the invites come from reliable friends.
LinkedIn looks like a very interesting tool for professional networking. Within a day of signing up, I have almost 20 friends / professional contacts on my list ! As I said, this site is suitable only for professional networking unlike Orkut, which is a fun place.
Do try LinkedIn and you might be pleased. Drop me a mail if you need an invite