A report implies a desire for an investigation. The police are not a hard drive where you store information. But, if you want to report it, call the police. Chances are it will be little more than a CAD entry (i.e. a dispatch note indicating that a vague and non-specific comment was made on social media).
What CRIME has actually been committed? I am not going to check the statutes of the two or three states involved, but, this is more than likely a misdemeanor at worst - and one that will cross state lines and, as such, not be followed up on.
Could WHAT happen? There is no way that a prosecutor could make the argument that the vague post "take him out" is a serious solicitation for murder. Perhaps you should speak with the facilitator of the FB page and ask that they remove the comment if they can.

