Well, it's like Facebook, aside of posting.
- You can chat with friends
- You can quick-barter games, cards and coupons between friends.
- You can see current friend status (XXX playing YYY).
- You will receive notices in friendband (XXX got achievement, YYY placed screenshot, ZZZ recommend game).
- You can see what missing cards from your badge friends have.
...Hmm, that's summarize it all, I think.