Totally inexperienced opinion here:

We're in an advertising bubble. Remember the ridiculousness surrounding the "Yo" app? People running around in circles about how it can be a great way to increase brand awareness due to the simplicity of it all? How it can connect with so many different apps? Yeah ... it's panned out exactly like its detractors predicted: sharp declines in downloads, hype is dead, and people have moved on to the next social app fad.

Another factor I'm noticing is international money. Uber didn't start approaching ridiculous valuation levels until Chinese investors started getting on the train.