Yik Yak exploded in popularity in 2014 and even had a $400 million valuation. It shut down in 2017 after a rise in bullying, harassment, bomb-threats, etc... and users moved elsewhere. It's now back under new ownership with an updated look and new rules. As before, users can post anonymously to people in a 5 mile radius, and now there's a zero tolerance, one strike policy for bullying and threats. It's currently available only on iOS in the US store but will expand to other regions and devices soon. Link
