It is the most useless app I've downloaded on my phone. Every time I've ever tried to use it I felt like I had completely wasted my time.
There are a lot of users but each new user doesn't bring that much to the experience. In fact, beyond a certain number they just increase the likelyhood that something negative is going to further pollute the community space.
So, not only is it worse that useless, its main metric is actually functioning backward compared to any other social media app.
There are a lot of smart people throwing a lot of money at this thing. I'd love for one of them to explain the reason because I just don't see it.
Edit: I thought of the perfect metaphor just now talking with my friend who'd never heard of Yik Yak and trying to describe it: It is like reading your local bathroom stall. A half a billion dollars worth of stall walls.