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.