Much of the competition in social networks are zero-sum games: when one entity wins, another loses. This is partly to do with the needs of those within the networks to display the symbols, and use the tools of their tribes to participate and be included. Facebook apps are no exception. The interesting thing about Facebook apps at the moment, is the competition between SuperWall and Funwall the two biggest apps on Facebook. Amongst my friends there are a few people who have both (in addition to Facebook's own Wall app that comes with your account), but most are active with one or the other. The charts from Adonomics shows how close the battle is between these two apps, from rivals Slide, Inc, and RockYou! Funwall is the blue line.
Daily Active Users
Total Number of Users
FunWall is currently number one because it has a higher proportion of active users, but SuperWall beats it on total users. They're not exact substitutes for each other, but in a world of limited attention, and network effects, one is going to start pulling ahead of the other soon, by eating into the other's social graphs.