For me there are 3
Facebook: For personal networking. I have friends all over the world with out FB no chance. Also without FB I would not have got back in touch with so many school friends, uni friends, and kids that used to live on my street.
LinkedIn: For professional networking. Wasn't so useful on this front 6months ago but as more and more people have been signing up (in the UK) I'm finding it increasing useful in engaging with colleagues, peers, suppliers, vendors etc. Also the application has improved quite a bit.
Twitter: For professional networking and research. Again in recent weeks months twitter I've been giving twitter a go and finding it increasingly useful in finding people using technology I am interested in, and following what people may be doing right now with such technology. Again more and more people are giving twitter a ago. In summary all of these tools are only as useful as the user base that is relevant to me. And how much they interact. It is probably worth mentioning that I have been using online social apps such as IRC, internet chat rooms, IM, and forums since around '94/'95 - geek!. I could chat all day about why Facebook is better than Hi5 or MySpace but that's for another time maybe.