Social media platforms are not a great place for discussion or community building. Social media platforms are great for broadcasting a message with a bit of light interaction. “We’ve got this new thing, tag a friend that would like it”, “What do you prefer thing A or thing B”, “Wasn’t the world a better place when you were 12”,etc. etc.
Forums on the other hand tend to be a deeper and broader discussion. My hobby community that I run is about old modified cars. There are also plenty of facebook pages and groups for these things. Often people will share a picture of a car with little to no context, the different behaviours between the different media are stark. On the facebook post you will get some likes, and a bunch of people tagging their mate who may also like it. On the forum you will (usually) get a source for the picture tracked down, more photos of the car and occasionally (if they exist) blogs and videos about it, sometimes the discussion will spin out into other similar cars or technical aspects of the build.
I’ve also found that nearly every facebook group I’ve been a part of from car things to music to my local town’s one is nearly always dominated by one or two voices. Obviously forums have a predominant posting caste, but it is usually more than one or two people even on small forums.
The inconsistency of message also makes facebook terrible for community, if I put something in a group or a page I run there is no guarantee that people will see it. What is worse is that it is also opaque, at least with a forum thread I can usually get a view count and maybe then I can understand if my title is working, or if I put it in the wrong place to engage people.
There is no separation of self on facebook so if I’m a member of a knitting group and a BDSM group my friends and work colleagues can see this. There is a lot to be said for the different people we are being separated.
So what has facebook got that forums haven’t? Ease of use, particularly in the mobile realm. A critical mass, as you point out you are trying to slice out an audience from a pre-existing set of users that are engaged with the platform. Light interactivity, people are often happy to be told what they like it seems, so no further discussion needed.
There is evidence though that the next generation of facebook users is not so engaged with the platform, or even joining it in the first place. Anecdotally my nieces and nephews have accounts but barely use them beyond occasionally resharing something from tumbler or updating their bands page with gigs
It seems youtube has really hit its stride with the next generation and how do one of the premiere production companies choose to make a community? :
Instead of writing all this (on my phone no less) I could have given a flippant single word response. Reddit.
You can dress it up how you want but Reddit is a discussion forum platform. Even worse it is a threaded discussion platform, like actual Usenet! It adds a novel(ish) way to bubble up good content but at its heart it is a discussion forum.
I should stop ranting here, but there is definitely a part 2 floating around in my head now I’ve released the broad strokes. One other thing I would say in defense of forums, you wouldn’t have got this reply on facebook
Ps. This forum/page isn’t super mobile friendly, I ended up writing this reply in a different app and posting it.