"Part of the problem with the bevy of cheap, easily available chat apps is that it's hard to get various social circles to agree on the same one. Four years ago, I personally laughed at a friend who tried to get me to install WhatsApp because it allowed cross-platform iOS-to-Android chatting. Now, I have several messaging apps installed, and I talk to disparate groups on each. For one group of friends, and different subsets of that group, I have WhatsApp chats; for another group I Facebook-message; another contingent is active on Snapchat; another I text. There is precious little overlap between them."
"It would not be surprising for Facebook to offer not just its canny tactics for conquering mindshare, but the tendrils of existing accounts that it already has in place for most of the connected world. Those tendrils could be a distinct advantage to the crowded and largely homogeneous messaging app market. It's an edge that none of them really have, a way to easily build when their competitors are so similar and when popularity spikes across demographics are difficult to control.
The need to communicate draws us to where the people are that we need to communicate with (see historical precedent: e-mail, AIM, SMS, Gchat). Facebook is already, arguably, that place, but it seems safe to say it hasn't become the chat powerhouse WhatsApp has."
Sums it it up nicely.