I put the most effort into building up a following on Twitter, and so far it has paid off hugely. So far, I haven't really knocked myself out on the other services, and i haven't seen much return - except for friendfeed which seems to be really good for SEO.
I'm curious - other than Twitter, is anyone having great success on any other social networking sites? As in, a lot of targeted customers, people regularly buying from you, etc.?