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.
Actually, Google does have an app like that. Search, Gmail, Calendar, Google+, etc - everything except for Maps. It's called Google Search, and is a shell of sorts for web versions of many of their apps, but you can also link the apps to it if you so desire.
You'd find that weird, right? Because you are forcing a significant change of behavior. Similarly, facebook breaking their core functionalities into specific apps is a significant change in how we already think about an existing product. To access Facebook Messages on my desktop, I do not visit messages.facebook.com or FMail. To chat, I don't goto chat.facebook.com. If I did, these app break ups could perhaps make more sense.
I consider myself tainted data point because I am a huge user of facebook on both desktop and mobile(app, not browser). I probably do not use facebook in a similar manner as a mobile only facebook user. They seem to be betting the company on mobile-only facebook user. Even then, their present strategy makes little sense to me because even when I am on mobile, the app switch causes friction and makes me use WhatsApp instead of FB Chat app. Why? Because I know WhatsApp has more reliable chat; and if I must switch apps to chat, may as well switch to the better one.
Bottom line is that with minimal real-estate, at a certain point there's diminishing returns on packing in more features while sacrificing simplicity.
In my opinion it's the obvious move for FB, and they're executing it well.
I think we can agree that most users won't fill their home screen with multiple Facebook apps. If they are moving the secondary FB apps somewhere else, be it a folder or another screen, there's a chance they might move the main FB app too.
I hope I'm wrong.