I think one of the major failures of social businesses on the Internet is that their business need for massive user growth forces them to upend basics of social interaction that we have taken for granted in the real world for eons.
If you create a new subreddit, anyone can show up and participate. Good actor or bad.
Can you imagine if you threw a physical party and let every single stranger in the world literally walk in your front door, wander around their house, and do what they want? You'd be a honeypot for thieves and vandals.
On the Internet, they don't take your physical stuff, but they harm the online equivalent: your information and attention. Every popular online forum is likewise a honeypot for scammers, shady advertisers, griefers, and other malcontents. Why wouldn't it be?
The typical suggested option is massive policing, but that's really hard in a world a bad actor can don a near-perfect disguise (create a new account) in an instant.
I would love to see sites like Reddit and Twitter adopt a model closer to real-world social interactions: something like a web of trust where you need to be invited to participate in a space and where there is some level of vouching for you. But those kinds of businesses don't scale well so we almost never see them.
Another option would be to force something like real identities. The reason a bar can get away with just a couple of bouncers is because a bad actor can't as easily escape the consequences of their actions by shedding their identity. But, of course, requiring real identity gets in the way of good actors who use anonymity for good reasons (generally avoiding other bad actors).
It's a hard problem, but it makes me sad that almost every business just ends up doing "let everyone in and accept that there's going to be shitbags all over the place fucking it up for everyone".