I came into the game too late to see the days that FATEs brought many into the open world. Certain FATEs such as Odin, Behemoth, Svara and so forth are almost always ran when they pop, but the more frequent notorious boss FATES that require multiple participants almost always expire. However, you can goad other players into doing them if you can round up 4-5 people to start them up. Players will join in if they see others doing it. This let's you know that there are still players in the open world. Just widely distributed throughout an area.

That said I would love for FATEs or some other kind of content to bring more together in the open world. It's difficult though when the rewards most people seek lie elsewhere. Perhaps that would be start.