I have a lot of Facebook friends that I haven't talked to in years. A good heuristic for whether we're still close is whether I feel comfortable texting them a happy birthday message (I hesitate when I worry that would feel too intimate).
I wonder whether there is an appetite for a social network where your friendships fade if you don't check-in regularly, which is what happens in real life. Instead of limiting your number of friends (e.g. Path), you would lose access to some info on friends' profiles if you hadn't visited their profile recently or if you hadn't exchanged messages. The intent would be to limit your friends to the people that you feel close to.