Facebook app, Messenger, WhatsApp, Instagram all feel a bit repetitive in features. Like they're competing against each other in many ways. WeChat as a single mega app feels very unified. If Mark builds the next WeChat for the West, I'm curious to know how he will unify those apps.
Just fyi, unifying Messenger, WhatsApp, and Instagram is something they mentioned a while back [0], and is something that they're moving foward with now [1]. Although it's looking like they're just going to get the underlying architecture the same so you can message people across apps and less that there is going to be 1 app for everything.