Yik Yak
An anonymous feed that only showed posts from people within a few miles. An experiment in how badly things go when anonymity is combined with proximity.
The app launched in 2013 and took off explosively on American university campuses, where the radius happened to coincide almost exactly with a student body. That was also what became the problem. In a small place where everyone knows who is nearby, an anonymous comment about "the girl in the third row of the lecture hall" is not anonymous to the person identified — it is maximally targeted. The company was valued in the hundreds of millions of dollars and then collapsed under a combination of sustained harassment, bomb threats that led to arrests, and universities blocking the service on their networks. It shut down in 2017 and relaunched in 2021 under new ownership with stricter rules.