As I said, I consider the invite system a plus, and invites can be customised. One server I am in has a permanent invite link on the website of the scholarly organisation they represent. Even with that, the amount of spammers and what I would call casual users (the ones that just wander in on Facebook groups and don’t understand that the group has a specific goal and represents a specific community) is near zero. Other groups are more restrictive, which further depresses that number.
As for the chat format: I fail to see the difference to somewhere like this BBS to be honest: there are different channels for different topics and within them people talk asynchronously. You can use it more real-time than a forum like this because you don’t have to refresh when someone else posts, but most servers I’m in are decidedly asynchronous with occasional bursts of side conversations breaking out when two or more people discuss something and happen to be online at the same time. But that happens here, too!
That’s exactly what I want! I want a closed-off backroom where Dave from Wigan and Carol from Omaha don’t barge in when I discuss (as an example) experimental archaeology or post-medieval earthenware because Facebook steered them into a group that was originally set up for professionals to exchange experiences (both real examples of topics but [barely] fictional people). That might sound elitist, but such spaces are necessary.