This will generate tons of noise for law enforcement that they cannot effectively find the true signals of those intending to commit violence as opposed to those that are just angry and like to vent online. There are just too many people posting angry stuff on social media these days to find the actually violent people. Whenever horrific school shootings happen, the media immediately looks at the attacker's social media and wonders why law enforcement didn't catch them ahead of time; this is why.

And since a crime hasn't yet been committed, there isn't much law enforcement can do to obtain a warrant (at least up until now). And even if they find someone that they suspect might commit a crime, they can't just hold that person indefinitely. They either need to have enough evidence to prosecute or let the target go.

Suggestions like this don't actually solve the problem, they just shift the blame elsewhere.