Everyone's threshold will be different, but for me, a community is lost when it's impossible to avoid the fundamental attribution error.
For example: I might get into a mood. I rant, I take the least charitable reading of a situation, I move away from people. I know I'm not always like that, I just get like that sometimes, and I move on.
When someone else does it, if I don't know them, I might think "Wow. White t-shirt brown shorts is an asshole. Stay away from them." See what I did? I said they are an asshole, not that they're in an assholish mood.
(Side note: This is one reason E-Prime, an English without the verb "to be", exists.)
The cure for the fundamental attribution error is knowing the person in multiple moods. Some people, OK, are snippy little shits who will self-aggrandize and look for every opportunity to start a fight, but most people have ups and downs. Knowing a person means seeing beyond any given mood or action to the person beyond the moment-to-moment.
Make a group big enough and there's no effective way to do that. You can't keep track of 50,000 names. You can't keep track of 50,000 unique images attached to names. Everyone fades into the mass, unless they're Power User level celebrity scale, and then they're reduced to a few broad strokes in your head. All you have to relate to them is what they're doing at the moment. The fundamental attribution error is the entirety of your social context.
That means you can't give anyone the benefit of the doubt. Are they arguing in favor of an unpopular opinion? Downvote and move on. They're an asshole because right at this moment they're taking the asshole's part. The biggest voting block wins, and the space is a hive mind, since there's no way to make a good argument for an unpopular position and have it be heard, whereas bad arguments for popular positions are promoted.