Comment by ethbr1
1. They've developed and open sourced incredibly useful and cool tech
2. They have some really smart people working there
3. They're well run from a business/financial perspective, especially considering their lack of a hardware platform
4. They've survived multiple paradigm shifts, and generally picked the right bets
Among other things.
0. Many people use Facebook messenger as primary contact book.
Even my parents are on Facebook messenger.
Convincing people to use signal is not easy, and there are lots of people I talk to whose phone number I don't have.