Value is a misnomer. There's no inherent value in 90% of the stuff that blows up. What's the "value" in Facebook? It's even hard to see today, think about how hard it was to see in 2006.
"Value" is the same nonsense idea as "solving a problem" that startup wantapreneurs are so obsessed with -- what problem did Snapchat really solve? What problem did FB solve that Myspace didn't? What problem did Google solve that Yahoo didn't?
Sometimes it's just about being different (Pinterest), being controversial (Snapchat), being novel (AirBnB), and having fun (reddit). Sometimes it's about cleaning up UX (Facebook), or making things just a bit easier to find (Google). But above all, it's about the right place, right time, and right execution. Fortune favors the bold.