As a newbie I frequent all of them. As you say, Rennlist is good for searching, and here as well, but a lot of it is old (I know some things don't change.) FB is at least up-to-date, even if it is wrong

Some stuff on FB is good, from my perspective, and I save the posts I want to follow up on. I watch a lot of the 944 content on Youtube, and IMO there are very good contributors for someone like me, but it's not a forum.
I have appreciated the help I have received here from the regular few who contribute. You have been there and done it several times, plus you have pushed the envelope of what these cars can achieve. But the PCGB forum is hardly buzzing - even the 997 group is quiet by comparison to other forums.
There is a gap for something better, but it needs a larger number of users than there appears to be. What are you looking for? It would be interesting to know which FB groups you follow.
These are mine for 944 in no particular order:
No Snob Porsche 944 Squad
Porsche 944 Turbo
Porsche 944 Turbo Owners (Uk & Europe)
Edredas Porsche 944 Group
Porsche 944 No Spam
Porsche 944 924 Spares
Porsche Club 924 - 944- 968 - 928
944Fest Message Board
Waleswood 944
Porsche 924 & 944 Parts