These are 4 shorcuts to bookmark the post on "social bookmark" type sites.
Delicious is a really cool way to keep track of bookmarks if you use different machines. Also a great way to share your finds with others.
Digg is more of a social news site that I browse everyday. It and Volnation are my two main stops. Basically, users submit stories they find and everyone votes (diggs) the stories they like and those stories rise to the front page. Originally it was just a tech news site for geeks like me, but now they have a lot of categories.
The other 2 I'm not as familiar with, but they are similar social "web 2.0" sites.
I could kinda see having the delicious link, if there is a thread you want to bookmark. But I have no idea why Freak thinks people would want to digg a VN thread.
edit: And actually, it didn't link to the post when I tried it on del.icio.us, just a link to the thread. If that's the case you could just have 1 set per thread, no?