There is no question that posting on Social Networking sites has a big influence on SEO for commercial sites.
At the moment, those social links are bigtime influences, because of the way the crawlers work, and because it is difficult to see how the crawlers results could be modded to weight the links without ignoring them.
If you want to benefit, here's a list of sites useful for SEO without paying consultants to put plugs up for your site:
BallHype,
Yahoo,
Buzz,
Digg,
Xmarks,
StumbleUpon,
Bleacher,
Report,
NowPublic,
Delicious,
Twitter
and there are a couple of others, but you don't want me to give EVERYTHING away, do you?