Philip Greenspun’s Weblog » Why Facebook is so addictive: the Like button:
It’s hard to exaggerate how much the “like” button changed the psychology of Facebook use. What had begun as a passive way to track your friends’ lives was now deeply interactive, and with exactly the sort of unpredictable feedback that motivated Zeiler’s pigeons. Users were gambling every time they shared a photo, web link, or status update. A post with zero likes wasn’t just privately painful, but also a kind of public condemnation: either you didn’t have enough online friends, or, worse still, your online friends weren’t impressed. Like pigeons, we’re more driven to seek feedback when it isn’t guaranteed.
Note, this is actually a quote from this book. Mostly linking to Phil to give credit for turning me on to this book and quoting it.