A reasonable analogy would be someone who didn't accept texts and insisted on party invitations (and communicating) through post or in-person visits. It's easy to say "Well a real friend would make the time to drop by", and that's perhaps a consistent definition of "real friend". But the fact remains that there's a whole host of pleasant social interactions (many of which others would define as "real friends") that you'd miss out on if you only relied on more inefficient communications technologies.

I'm not a heavy Facebook user, and I've considered deleting it multiple times, but each time I couldn't convince myself that there was any advantage to doing so over just using it as an inbox. Particularly with things like the News Feed Eradicator Chrome extension, I can basically pretend that all the irritating parts of Facebook don't exist when I check my messages and events. Having a single place where all the members of a group can see and share information on their own schedules is infinitely more convenient than trying to hack together the same thing over multiple bilateral communication channels (i.e texting/calling). Sure it would be better if it was federated, but given that there is no such widely-used solution, it's not hard to understand how people refusing to participate in FB groups can be fairly perceived as being the inconsiderate one (or else being left out altogether).