https://en.wikipedia.org/wiki/Onavo
TBH has been in the top 10 free apps for a few weeks now. FB has a good track record of buying small teams with good engagement numbers and letting them run themselves (Whatsapp/Instagram).
I'm sure Facebook looked at their numbers via Onavo and decided they were on a pretty good trajectory.
Also, it should be noted that TBH doesn't even have an Android app yet.
The question now is will TBH turn out to be more of a draw with friends (flash in the pan game) or have staying power like Bitmoji does.
Bitmoji was purchased by Snap Inc for around 100 million about a year ago. It has managed to consistently stay in the top 10 free apps since then: