To my mind the killer is event organisation. Put it this way: Meetup is apparently a viable business while charging money for a less-good version of something Facebook offers for free-at-point-of-use. No-one else really competes in that space at all (group chats aren't suitable because people who can't make a given event end up being spammed with all the details of it anyway, while people who join late miss out on details that were posted earlier).