In the eyes of Wall Street, it's all about growth. Facebook continues to be among the Top 3 or Top 5 apps in installed base but if you're not growing at the same pace as before, it's bad.
Facebook is obviously not likely to significantly grow their user base at this stage. There’s only so many people. Wall Street doesn’t expect them to do the impossible.
Or does it? Wall Street tends to do irrational stuff, and besides, there are still about four billion people in the world without a Facebook account, so there’s still room to grow.