My guess would be Grindr or one of its clones.

Loopt, colorful popped collars notwithstanding, is not particularly gay.

I'd have guessed Grindr, but this is saying that while the police press contacts don't know what app it was, it specifically wasn't Grindr:

http://joemygod.blogspot.com/2010/04/phoenix-man-stabbed-to-...