It's a crime to conspire to murder; to commit fraud; to arrange an act of terrorism; and so on. And in all relevant cases, social media was used in court after-the-fact just as evidence.
So we're talking about activity on social media which are crimes themselves, just being used as evidence after other crimes have been committed.
This is the problem with the propaganda being put out there at the moment, none of it is true -- and all of it is in the service of disgusing the content of actual court cases.
People on the far-right like to use the phrase "posting to social media" when they mean "using online communication platforms to arrange a violent riot with the intent to murder people". And they like to pretend this evidence collection is happening before those actions -- when its after, and presented in court.