Freedom of Speech and Support of Terrorism--where to draw the line?

First Amendment freedom of speech limits the creation and application of substantive criminal law.  See this controversial case of  a Maryland Imam.  

But in the last two years, Imam Bengharsa’s public pronouncements have taken a dark turn. On Facebook, he has openly endorsed the Islamic State, posted gruesome videos showing ISIS fighters beheading and burning alive their enemies and praised terrorist attacks overseas. The “Islamic Jurisprudence Center” website he set up last year has condemned American mosques as un-Islamic and declared that homosexual acts should be punished by death.

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Sebastian Gregerson Credit Midland County Sheriff's Office, via Associated Press

That is not all."  See the link for more details."The case poses in a striking way the dilemma for the F.B.I. in deciding when constitutionally protected speech crosses into inciting violence or conspiring to commit a terrorist act.'