For those who aren't familiar with Grindr, from their website:
Grindr is a simple app that uses your mobile device’s location-based services to show you the guys closest to you who are also on Grindr. How much of your info they see is entirely your call.
While it's described as a social-media app, it's primarily used by men looking to hook up for sex with other men. There's a version for opposite sex partners called Tindr, but it's a little more subtle, at least on the hooking-up-for-sex front.
A YouTube regular, Neil McNeil, asked a bunch of his straight friends what they thought of Grindr, and here's the result: