Social Point introduced a "Population Limit" in Dragon City. (~Sep 2012)
In order to increase your Population Limit, you had to pay gems. If you maxed the limit, you couldn't add any more dragons. You couldn't place any eggs from the hatchery, meaning if you had Eggs in there when they added the feature, you couldn't get them out if you had reached the limit.
In Oct 18/19, 2012, it was finally removed due to many people quitting the game.