@tobee I am a member of a few on FB.
They are mostly good natured, friendly groups, but as with all large groups, there can be some argy bargy from time to time and some snarky people. The main issue I have, is with such big groups, the same questions get asked again and again. The most common one is "Do you prefer Kindle over 'real' books?" - cue 100s of smug people piling in with "Oh no I love a "real" book, looking at a screen isn't reading", then someone will ask "does it matter?" and common sense is restored. Happens about three times a day, more at the weekend. Next most common question is "Are audio books cheating?" - never really understand this question, but I think it has something to do with people very focused on GoodReads challenges. Next is the "Have you read Crawdads (2020), Eleanor Oliphant (2019, Normal People (2018) ... you get the idea.
If you can get past these annoyances.....
The Facebook Book Club has over 80k members and they come from all over the world, very active, popular group but so big you rarely run into the same person twice.
Lost in a Good Book is slightly smaller at about 30K membership, I think it has more British posters, but there are certainly people there from all over.
I like Women Reading Great Books. Large group, 70K+ members. Very active, lots of types of books being discussed. Lots of American readers and there has been some heavy moderation over the election, but it is calming down a bit now.