Using Facebook's messenger app requires you to know someone in way which just having their phone number doesn't.
For example, if I'm going to go on a date with someone and we've swapped numbers that's enough for us to talk by SMS/iMessage/WhatsApp, whilst we probably[1] don't know each other well enough yet to expose our digital lives to each other on Facebook.
[1] - This entirely depends on things like age, social norms, how much personal information is on your Facebook page etc. Also, depending on are you going on a date with someone you already know vs. someone new. Mostly the point being that anything that just needs a phone number has a low barrier to communication while also not leaking any personal information (other than your phone number).
For example, if I'm going to go on a date with someone and we've swapped numbers that's enough for us to talk by SMS/iMessage/WhatsApp, whilst we probably[1] don't know each other well enough yet to expose our digital lives to each other on Facebook.
[1] - This entirely depends on things like age, social norms, how much personal information is on your Facebook page etc. Also, depending on are you going on a date with someone you already know vs. someone new. Mostly the point being that anything that just needs a phone number has a low barrier to communication while also not leaking any personal information (other than your phone number).