I haven't dealt with this exact situation but one where someone thought bc it was on the internet they were anonymous and could make threats about bodily harm, physical violence, etc. I ended up doing an DNS look up and was able to figure out where he lived and more importantly where he worked and that he made some of the threats from his work pc. When that was mentioned and that we would be sending a letter to his employer along with copies of the threats all of a sudden he disappeared.
Now in your case, since you've found where he lives via FB, google+ I suggest you let him know and that you will be contacting his friends, family, employer (if you can find one) as a starting point. It's been my experience from reading many of these types of stories over the years that most law enforcement agencies don't care or don't have the time to put into something like this which is very unfortunate as it only promotes this behavior.
