I think Stumbleupon, Facebook, Twitter, identi.ca are good site by using these sites properly you can generate traffic .
Yeah facebook and twitter are the most popular right now but in several years from now, there will be something else. I think myspace is still being used but don't hear too much about it anymore.
I agree Facebook is the best among all of them, But to get lot of traffic, we need to have presence on most of them. If we have human resoucre to be spend on Social Networks, we can split the work time by giving around 70% to facebook.. and rest 30% to all other networks total.... Try this ratio.. It works..
Facebook , twitter and stumbleupon are the top three according to me .These alone drive like 20,000 page views to my site in 4 days or 5
i think the sites listed below are best sites to get good traffic Facebook Twitter MySpace LinkedIn Friendster i5 Orkut