I was at dinner with a friend. Because of unexpected circumstances, we ended up sharing a table with one of her friends and some people that friend was with.
One of her friend's friends was quite gregarious and engaging towards my (non-romantic, by the way) friend. Seemed pretty obvious to me, at least.
We get back to my friend's place, and this guy is IM-ing her on FB Messenger with a bunch of flattering and engaging texts. She didn't give him her number, despite his hints in the restaurant parking lot, but he readily found her on FB through their mutual friend/contact. (Who, by the way, told her, "Yeah, no... He was a jerk to me and you don't want to go there.")
Sorry for the lengthy story. This friend of mine also went through an extended period of significant but not always close network building, this past year. And this is how much of that worked: Where phone numbers weren't shared, PM-ing through FB Messenger.
Messenger is well on the way to becoming the de facto "public" venue for IM-ing.
Other platforms tend to be turned to for more private, personal stuff -- at least among folks who actively use multiple platforms.
P.S. Invasive as it is, and with all the reservations I have, you nonetheless have to give FB credit (as a self-interested business) for turning Messenger into this. And, privacy concerns and limitations aside, it pretty much "just works". Which is the definition of win and first choice, for most "ordinary" folks.