Here in Northern Ireland it was Bebo until last year...now Facebook is king. Twitter is slow by comparison and most other sites not talked about at all (except for musicians and myspace). NI is generally that wee bit behind the times though!
I would say in the United States it would be Facebook, twitter, then it all depends on what you want out of the social network. Smaller ones are good for targeted traffic and larger ones for volume and pushing products.
IMHO, the big two are facebook and Twitter. You shouldn't only use one or the other, combine both to get a synergistic boost in traffic that will also help to build your reputation and credibility!
FB is the biggest around here, but twitter seems to be really slow to catch on compared to other parts of the world...maybe i should move lol