Social bookmarking is different from social networking. I got a really clear explanation of this recently. Yet, I have subscribed to many sites that belong in both categories and did not have a full grasp of the difference between the two of them.
Ok, this is my explanation:
Both are part of Web 2.0. However, social bookmarking is like choosing things to go into your favorites to go to later. Except there is a wide internet world that can see the sites you have chosen.
Whereas, social networking is like visiting sites you want to hang out in and play, visit, learn and share information with others in the world wide web who have silmilar interests.
Also, see and enjoy an earlier post on Twitter.
Here is a great, easy to understand video from Common Craft on Social Bookmarking.