I would but many people wouldn't because they don't want to be branded racists. That is what happened in the San Bernardino terror attack. The neighbors saw suspicious stuff but didn't want to be politically incorrect to Muslims.
HELL, the FBI did the same thing before the Fort Hood terror attack so it may not matter what you report at least if it has to do with Muslims.
"Report shows FBI ignored accused Fort Hood shooter Nidal Hasan out of political correctness"
Cruz was on the FBI's radar but the 1st Amendment makes aspiring to be a school shooter legal he didn't make any actionable threats. If the FBI arrested him all his lawyer would have to say is the kid was joking like MILLIONS of other young adults do every day. I don't think the entire FBI could police 4chan for morons much less the entire internet.
"An FBI official acknowledged on Thursday the bureau received a tip months before the Florida high school massacre about a social media comment by a Nikolas Cruz, who aspired to be “a professional school shooter."