The reward are those random bits of validation you get from friends that say to you in real life "I loved your Snap the other day" and when your friends directly respond on snapchat
Smart! That plus striking comment: "You can see the number of, and names of, the people who view/screenshot your Story."
That ocasional and random reward that make something addictive (like what keep you checking your email all the time, at last receiving that one useful/interesting email in the middle of several useless one). Contrasting to predictable Facebook likes that are measured solely by quantity.
I end with the impression that is a more genuine connection than the commoditized "FB like" and more personal than the "Retweet" and "FB share" that are performed by people you don't even know.
