Earlier quoted context omitted.
The answer is network effect and friction . It is hard to communicate to everyone on your friends list that moving forward they can reach you via email or text only. It’s going to work with close friends and family but other people that want to reach out will not be able to find you. And there are always cases when you want to connect (or be easy to find) with someone who is not a close acquaintance.
I'm not trying to be combative, but that still seems like a very weak reason. And it's one that I used to use, not just with FB, but Twitter, IG and LinkedIn. They all held the same promise and failed to deliver it. The idea that we need to be constantly networking is overblown, to say the least. When you step back and have an honest conversation with yourself about how much having access to these people you occasion…
Now you just passively absorb updates from people to stay factually informed but don’t directly engage with one another.
With email/sms, you can just ask somebody “hey what’s up?” And get their big updates. It’s more active and requires some more investment but that’s a good thing for making stronger relationships.
And for all those distant connections that you follow on FB but don’t want to talk to… you can ask your real friends “hey, have you anything about so-and-so?”