I think it creates value in the ability to keep track of your personal social network. Its not entirely difficult to manage contacts on your own, but I think its nice that at the very least I can send a Facebook message to someone (if not gather their contact information via their profile if they choose to include it) if I'd like to ask them a question or catch up with them. In the past, you might have to ask a mutual friend or spend some energy figuring out how to reconnect with them.

But I agree that their revenue is generated mostly because most of their users have trouble limiting their time on the site, which can be considered a negative impact.