Yo's popularity entirely rides two facts: (1) it's a silly app, (2) it got a million dollars. We currently know nothing about user engagement and retention. I'd be surprised if anyone uses it for more than a few weeks.
The hype and funding is based on the HOPE that something will come of it. However, retention is already a problem in Tel Aviv, where the app got its start. As fast as it has risen, it will fall (my prediction, anyway).