I was listening to NPR a few months ago and they actually had a bit on tracking Twitter for gang activity and the ethics behind scanning twitter. Here's a link to the story [1]. One of the products they outline is called BlueJay [2] and scans for key words and locations. I guess NPR isn't popular among gang members or they would probably stop.
[1] http://www.npr.org/blogs/alltechconsidered/2014/02/28/284131...