Very relevant paper: "Hardened dildo.io, A Cryptographically Secure, Usable Matchmaking Service"

https://courses.csail.mit.edu/6.857/2016/files/13.pdf

I've actually implemented this paper in a hackathon, but we never launched it. It's basically like tinder, but no central server has access to anyone's likes. Yet, by using homomorphic encryption and a calculation performed on the server (through which no information is revealed), you can know if someone you liked likes you back. It's pretty cool!