According to my blog visitors' usage trend, these are the most popular social bookmarking services:
Favorites, Twitter, Google, Live,
Print, Digg, Facebook, MySpace,
Delicious, Email, StumbleUpon, and AIM.
May be this trend will only stand very similar for those, whose content is technical in nature. For other type of websites also, these bookmarking services will be important, I guess!
This graph is directly from Addthis.