I'd argue that communication via email can work just as well as FB. It's a little harder to "opt in" to updates, but with spam filters and other email features, it seems like it can be a lot less noise to have to filter through. But this is coming from someone who doesn't use FB.
It works on a small scale with a group of people who regularly communicate with each other. That's how I organize most of my social events - e-mail a bunch of friends and say "Hey, wanna get together on Saturday?"
It fails when groups are larger or more loosely attached. In my cousin's case - my dad was one of 10 brothers and sisters (many with their own spouses), I have 16 cousins on that side (again with spouses), 3 half-cousins, 10 cousins-once-removed, 1 cousin-twice-removed, and there's a tendency for at least one person to feel offended if they don't get the news when everybody else gets the news. It's somewhat understandable that my cousin would want a broadcast medium rather than trying to remember all that.
Or as another example - a friend of mine died recently, and I found out through FB. I hadn't been in touch with her for several years, since before she got married, I'd never met her husband, and he certainly didn't have my e-mail. Still, I appreciated knowing, and passed on that info to other mutual friends, who also appreciated knowing. That's the long-tail; in my parents' generation, they might've found out at some reunion 30 years in the future, long after the funeral has passed and people are done sharing memories & photos.
The problem that Facebook solves is finding a way to contact someone when all you know is their name. If you don't have a phone number, address, or email but you know First and Last (or even First - if you have related friends), then Facebook still works to get in touch. Whereas phone books for personal numbers aren't really a thing. Facebook is that phone book.
