I know I'm probably in the minority here, but I really like Facebook's decision to create multiple apps. We aren't good with thinking of apps as having multiple functionalities. Our thought process is more along the lines of "This app is for this, this app is for this, etc."
Having one app means a lot of features get lost in the app, and people turn to alternative apps (like Foursquare, Batch or WeChat) for functionality.
Edit: To put it another way, what if Google had one app and inside it was GMail, Search, Maps, etc? Facebook sees these as products, not features.