Earlier quoted context omitted.
... and LINE, and Kakaotalk, and ... The thing is that many of these apps are both pretty good, and have insane market penetration... It's hard to see why anybody would switch to a FB app that's very likely worse if they're already using something else (so all their contacts are there).
If they're on Facebook all their contacts will be on Facebook Messenger. Does Facebook have poor market penetration in Asia?
I do not have numbers, but while a fair number of people use FB in Japan, it's far from the juggernaut it is in the U.S. According to Wikipedia, Japan has about 17 million FB users, Korea about 10 million, and China only about 0.6 million!
LINE, in the meantime, just celebrated its 300 millionth user, and its user base is concentrated in Asian countries.