That's a commonly-held belief, but runs contrary to the history and law behind the quote. That particular quote was an example in a US Supreme Court case that was decided poorly and later changed (in 1969 I think). In the US, I believe threats of violence or harm are covered by the First Amendment, unless the speech "is directed to inciting or producing imminent lawless action and is likely to incite or produce such action". So the speech has to meet several qualifications to "not be free speech". What those people were doing on Facebook in Brazil would very likely be considered protected speech in the US. Not that what applies in the US applies to Brazil, but since the quote was from a US case, I figured it was with commenting.
Here's a good article about the quote's history: https://www.theatlantic.com/national/archive/2012/11/its-tim...