Any kind of system where you can translate http requests to money will have a problem with fake users. Might be click fraud for ads, sending spam to web forums, or liking YouTube videos to get the videos recommended.

For some of these operations, you can just work off of the content. Spam messages need to advertise something, so the text needs to look very different than for legit posts.

But something like an upvote or like? It's a single bit of information, you can't say if it's legit or fraudulent in isolation. So then you need to come up with additional signals to cluster on from wherever you can.

Some of it will be behavioural (these 10k users only liked these spammy videos), but a lot of it has to be environmental.

Posting blatant spam message got removed by NLP pipelines, spammers evolves to be more implicit, for example, the avatar of the user contains an image of crypto currency ad, then the spammer "liked" your video. You got a notification, and you noticed his/her avatar.

Had to OCR all those god damn avatars.

I can confidently say, having rubbed shoulders with the botting / gold farming / hacker community in China that it is HUGE. Like YYYUGE! back when the whole ride hailing industry was still in a subsidy war, you had drivers who were spoofing their rides to get kickbacks from didi/Uber. Pinduoduo suffered a pretty big loss a while back from not accounts scalping subsidies. I have friends in my wechat who used to share their huge walls of remotely controlled phones. So yes it is a huge problem in China. There is even a phrase for it, 撸羊毛

I worked on a social platform some years ago, when it the population was somewhat 'mature' after inital growth spike, then the majority of new signups were fake. "Cleaning up" fake accounts was a big part of the job for the customer service team, as well as automation tools to suppor that task.

If visibility on your platform is somewhat commercially relevant, then you will have lots of people pushing fake accounts for various goals. And if you ignore them, then the more technically competent ones will set up offers to sell access to fake accounts on your platform, so that they will be abused also by actors who don't have the ability to create thousands of fake accounts on their own.

It's not a problem if your platform is not profitable. Or the crowds is not worthy to spam.