You will be requiring a cybernet police that surf the world wide web for criminal activity. But since your friend or you are harassed in what way? There are different kinds of harassment, and I donot think this is a crime per se or could be describe as a crime itself. Crime are those victimized by syndicate that uses the internet for asking money or selling drugs through the internet. Harassment is kinda hard to describe here and since your friend is the one being harassed why is it you are the one complaining about it? He or she should be the one opening up this complain as the one being harassed that is why I ask if it is your friend or it is you.
Chance of being arrested in the US just for this? Are you kidding me? Facebook is a language confine inthe internet, you have to require a permission to the authoritize if a crime committed in website can elevate itself in court. No case has ever prosper yet in this type.
You should ask yourself first if the person committed a crime or not. That is the very first question the authority will ask of your friend. Now if your friend said yes, so what is the crime or what this guy done to consider the thing as a crime? Did he rob your friend, molest, sold drug etc....none I suppose so what is the crime? Harassment? on what degree that your friend is suffering from mental anguish or in such a way your friend is mess up physically or mentally or emotionally? None again. So you people are just wasting the time of the police or the authoritize demanding to arrest such a person in the US.