Major social media sites like Digg and StumbleUpon get most of the attention, but smaller, more targeted niche sites can provide excellent opportunities for networking with others in your niche and getting some exposure for your own work. While others are chasing after the Digg front page and 99% are getting nowhere, you could be getting hundreds and thousands of higher quality visitors over the same period of time by targeting niche social media sites. Read the full article
Don't forget beta.stackoverflow.com - this one's not available without a beta invite yet but it should be a nice SN for software developers when it goes live.