Re: Facebook and Facebook messenger
I don't think that you really need both apps, but what it comes down to for me is integration. It's a roundabout way of doing things and maybe it will become more streamlined in ios7, but if you go into your contacts app and click on someones Facebook IM link it will launch Facebook Messenger to that contact so you can send them a quick message without opening the entire facebook app. Likewise if you just want to see what your friend John Doe is up to, and you have no desire to get lost in your newsfeed or timeline you can click his Facebook Page name and it will launch the Facebook app directly to his wall.
Originally Posted by theKHMERboy
It is a little convoluted at this time as to how all this works, or why you would even want to go through the hassle, depending on how many contacts you have of course, but the functionality is there to built upon later if Apple sees fit. Of course having said this not having all of your contacts from Facebook syncing over to your phone, as has happened in recent releases of the Facebook app, completely negates the usability of this design choice.