Email, text messages, the telephone, and a million other similar services?
I will point out that the parent seems to be referring to the posts that people make to no specific recipient. That is not keeping contact with your friends, this is watching your friend make an appearance on TV, so to speak. Admittedly neat the first time you see it, but the novelty quickly fades when they are "on TV" every day.
Yes, Facebook Messenger and Groups do actually provide a decent service for keeping contact with friends around the world. You can absolutely use Facebook in that way, but, from the perspective of business longevity and health, does it really provide more than similar services that do the same?
Personally, I'm not sure the network effects are as strong when talking about friends keeping in contact with each other. If I want to check in with a close friend who lives abroad, it is not a huge barrier to ask for their phone number or email address. And chances are I already have that information.
The one value proposition that Facebook does have, in my option, is that it is the internet's "telephone book". For acquaintances that you want to contact and have to look them up to find them, it is a pretty good bet that you will find them on Facebook. But there is also plenty of competition here as well, such as Linkedin, which seems to embrace the "telephone book" concept even more so than Facebook.