The early iterations of Facebook, just like the early iterations of most internet community and social platforms legitimately brought and kept people together and helped them forge and maintain more solid group connections… I still communicate with a wider range of people from my past, high school, university, etc because of it and enjoy easy access to excellent information about certain vehicles and natural areas (hot springs) because of it. That core value still exists behind all the evil.

Anecdote, I know plenty of people who use FB exclusively for private and secret groups. The ability to require approval before approving (tied to looking up the profiles of people applying) while still allowing members to ask questions anonymously within the group (like maybe for health advice), moderation, some basic administrative roles, event invites / rsvp, discoverability by location (new mom club around me).

Not like any of these features are particularly good, but the combination beats anything else out there, which is surprising because usage of groups seems to be huge.

Discord comes close, but the complaint I see from FB group users is they don't like the real-time chat stream.

Before Facebook, after people lost touch with their high school or college friends, they often didn't have a good way to get back in touch again.

This is harder to do accidentally now, for better or worse.

But I suppose there are lots of other alternatives nowadays.