Uhhm... I'm wondering how you reach the conclusion that the police don't trawl social media looking for things. This is
precisely what they're doing. Maybe keep up with current events!
It's a gold-mine for every law-enforcement service on the planet, for good or ill. They drop to their knees every night, and thank God for Twitter, Facebook, Parler, etc.
And, you might be partially right -- it'll only work for rounding up the really dumb ones. All the smart ones are already on Signal, Telegram, ...
But, seriously. This has got to end.
Throwing a kid into juvenile detention because they yell at their friend "I'm gonna kill you!" in the middle of a game or make a gun shape with their hand at school has got to hold different weight than someone posting an instructional video on beheading. But, no, you have kids kicked out of school because someone saw a toy gun on a Zoom call...
The real problem for police is that they have 5% of the population being called "Racist" or "Terrorist" or "Nazi". Not 0.001%. They can't possible track down any real risks.
However, a social network using K-means Clustering easily isolates the "everyone other than me is a racist" crowd, from the "we're normally quiet and tolerant, but holy smokes this guy has really lost the plot" crowd.
The police could use that kind of help.