[Ankesh]

Thanks Chris.

Not recently - but some time back I had run a rudimentary test. At that time, giving just 2-3 options of social media sites worked better than putting up a consolidated icon like AddThis or ShareThis.

I would say - focus on just 4:
Twitter
Facebook
Digg
StumbleUpon

Unless you are a niche site - you probably don't want to add any more options than the above 4 on your site. I would probably cut down StumbleUpon button too - doesn't help much (in my tests at least - SU only measures the votes from the toolbar - not from websites...)