After seeing an article on the app at CNET and out of sheer curiosity, I went ahead and installed it. Disclaimer: I only have access to the iOS app, so I can't attest to how it works with Android. I also have no interest in hookups - I just wanted to see if it was all news hype or if people actually used it.
After opening the app and connecting, I proceeded to carefully swipe left and right to go through the list of friends. Swiping up indicates you're up for hanging out, swiping down indicates you're "down to bang." I didn't want to be sending any false signals.
After going through about 20 people, I felt absolutely dirty at the thought of banging any of the people whose pictures came across my screen. It quickly became apparent that the app just goes through your entire friends list, not just those friends who also installed it. For me it started by showing everyone I friended while I was studying abroad. After that, it showed people in no particular order. Married people, single people, people with kids, even relatives I haven't specified as relatives on Facebook - it will show them all. You can have it show either guys, girls, or both.
If you installed the app after January 2013, its privacy setting defaults to "me only". Nobody else will see you've installed it. I tried to do a Facebook search to see if anyone on my friends list made it visible - zero results. So really, there's no way I could verify if anyone has it installed. If you actually do want to use it to indicated you want to bang with a particular friend, you'll have to swipe through a whole lot of people to get to that person. There is no search function that I was able to find.
I only had the app installed for about 20 minutes, so I can't say conclusively if it's a dud or not. There are no people on my friends list that I want to bang anyway. The app had 29 reviews in the Apple app store compared to 970 for Grindr, if that indicates anything about its popularity.
My conclusion: this app gets a "McKayla is not impressed."