An MMO's comunity will eb and flow with time. That's the nature of an MMO. The Trick is to make sure they return after trying that new "HOT" game. The core players, the ones that don't leave, or keep thier accounts active even while trying a new game, are part of the reason people come back. They pull thier friends back in after the shine wears off of whatever pulled them away. But even the core wont stick around if the the surver population is so low it seems like nobody is around. So a merge is a good way for loyal players to continue to play, and enjoying the game. Thus building a solid comunity for returning players to come home to, and decide to stay.