The trick is other people that your people care about. Other people in the same place as your people. Other people who aren't going to leave. It's very easy for a group of friends to persist beyond a given environment. We've all seen it happen.
Friendships always migrate out of the game.
If you rely on other people to keep folks in your game-you're gonna lose. This is why the parlor game sites have virtually no customer loyalty and are casual in more ways than one. There's no emotional investment there-and if there is, it's all too easy to drop that friend an email and interact with them without having to sign up for yet another tedious game of hearts. People they can't take with them. Identity they can't take with them.