Poker room selection: most online poker players play at several poker rooms these days. This carries several advantages. Bonus hunters for instance will sign up to a poker room and only hang around until they manage to unlock the sign-up bonus. Once they're done, they move on and sign up at a different room where they start the whole thing over.
If you receive a rakeback deal at a site though, it no longer makes any sense to move on just for a flimsy sign-up bonus. Maintaining accounts at different online poker rooms makes sense from other angles too: players will be able to play against different opponents and different styles this way.