We are experimenting with these "social networking" buttons as a way to get some backlinks.
Incoming links help us gets ranked in the search engines. If you have a digg account, and you digg a post, then we automatically have a backlink from their website. If others also digg it, then it brings greater attention to the post which in turn helps generate links.
If you look around at some other forums you will notice that they are becoming more popular.