The main app is optimized for reading your timeline. Fetches notifications, friends, messages etc.

It is hard to optimize for different tasks at once, while I'd also argue it would be possible to strike a good balance. But I think it is a pragmatic decision rather than try to achieve that perfect balance, and let several instagrams take over, do the right thing for each task and a year down the line knowing usage patterns, with much better hardware, you can think about a consolidated app.

I completely agree. Facebook's iPhone app experience is overall horrible in my experience. It just simply rarely works.

I think they should fix it. But if their current solution is to make the photo taking/sharing a separate app, that's fine with me. I use Facebook mobile more to take/share photos than anything else on Facebook.

They started off with that approach, the app had a pseudo home screen with quick access to different functions.

Ultimately I think it boils down to what you think of when you want to do take a photo or send a message which maps to (Facebook Camera, Facebook Messenger) more naturally than just Facebook.