A sign of success for an MMO is it's first expansion. It opens up new features, new abilities, but the most notable thing is often the world increasing in size.
That works early on and while the game is still increasing in popularity. However, after the game has peaked, expansion results in less and less people per zone. This effectively turns a large portion of the game into a single player experience, which is traditionally a weak point in MMOs.
Combining and/or linking servers can help to hide the issue for a while but ultimately doesn't stop the majority of the game from becoming a series of ghost towns.
In my opinion, this a major cause to the decline in traditional, long-running MMOs.
Do you think this is a major issue? What could be done to prevent it?