I pretty much exclusively use Facebook and I run a group on Celtic coins there with over 3,000 members (https://www.facebook.com/groups/2375591872472089). I'm a member of quite a few ancient coin groups over there.
CoinTalk was good to read but not worth posting on, and I find Forum Ancient Coins too visually unappealing to look at, so I seldom go there. I’m trying to stick with this forum for a while, but I find myself only checking it every couple of days. The problem I have with the forums is that you only get to see the thread title to work out if it’s worth reading. A longer abstract would be useful.
I tried reddit and discord but I didn't really like them. I try every now and again to engage with them, but it never sticks. I suspect it’s because they don’t offer me anything I don’t already get from Facebook so there’s no great incentive to open up another page or app.