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.