Gamasutra: Game designers have a term that I think is about to see a lot more use than it ever has before. The term is "stickiness"; and what it describes isn't nearly as gross as you might be thinking.
"Stickiness" is a property of multiplayer games; and in the past it's mainly been applied to MUDs and MMOs. The idea is this:
1.A player begins playing an MMO.
2.Over time he gets to know some other players in that MMO.
3.Eventually he joins a guild or otherwise becomes part of a community of players.
4.After a while of this, he no longer plays the game so much because he finds it fun, but simply because it's where his community is - he cares about being part of this circle of friends more than he ever cared about killing NPCs or completing quests.