I just finished the MVP and you can try out at https://nameless.im
You can create a chatroom that can only be joined with an invite code that you send to your friends to join. The idea is you know who is in the chatroom, but you don't know who wrote which message. You can require everyone to sign in with their Facebook profile so that everybody knows who joined.
When I first told my friends about it they either thought it was evil and will ruin peoples' lives and they will never use it, or that it was super exciting and they can't wait to try it. The best kind of feedback.
I initially had the idea two years ago and I built an iOS app that got rejected from the AppStore because of the way the app works. Turns out it can never be accepted if I really want to keep the messages anonymous.
Built with React on Firebase. And yes, the messages are actually 100% anonymous, no profile data is stored in the database.
I have no idea how to monetize it. I only built it to distract myself from another project that I should definitely be working on right now.