People seem to think social media is akin to private communications where it's more akin to the public square. Making your IG/FB/whatever profile private doesn't change that.
In NYC for example, there's been a large uptick in teen shootings, many adjacent to schools, and a lot of it involves the idiots posting on social media before & after. One tool could be simply scraping social media for these postings. Another alternate, pre-internet tool was stop&frisk.
While you have a constitutional right to not be searched without consent/probably cause, you do not have a constitutional right to spouting off in the public square without consequence. What you say publicly can & will be used against you in the court of law.
Putting out an IG post of yourself with illegal guns or inciting a shooting is no more private than printing out posters of the same and putting them up around the neighborhood.
Do you have a citation for this that doesn't just parrot an NYPD source?