I just wish that Facebook, Twitter, etc, were built along the decentralized, federated model of "classic Internet" things like the web and email. It troubles me that single companies are becoming the custodians of so much data about people's relationships. There are already plenty of points of control on the Internet (ICANN, oligopolistic ISPs, registrars, etc) and it would be nice if we didn't keep adding more. Most of the applications which made the Internet take off were developed without a commercial motive, but social networks' main concern now is turning eyeballs into revenue rather than sustainably adding to the architecture of the net.
As for the addictive aspects people complain about, who cares? I have a Facebook account (so people can get in touch with me) but barely use it. That part of it is entirely within each person's control.