I'm confused about this as well. Why are they choosing to compete with themselves?
You choose A, Facebook wins. You choose B, Facebook wins.
I believe WhatsApp and FB Messenger are the two top chat networks right now (sadly), at least when looking at regional dominance. And since the value of a network to the individual is dependant on the size, AND they're locked up, they win whatever you choose.
You might loose because you chose the network with less friends, but that changes nothing in the end.