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.
Anecdotes can go both ways. As far as I've been able to tell, WhatsApp is most popular outside of the US.
And it's even hard to spot a Java-applet based phone. Even the lowest classes just use cheaper or older Androids.
The whole end-to-end encryption on WhatsApp doesn't seem in the Facebook spirit at all.