We tweet, we like, and we share— but what are the consequences of our growing dependence on social media? As digital platforms increasingly become a lifeline to stay connected, Silicon Valley insiders reveal how social media is reprogramming civilization by exposing what’s hiding on the other side of your screen.

The personal data of more than half a billion Facebook Inc. users reemerged online for free on Saturday, a reminder of the company’s ability to collect mountains of information and its struggles to protect these sensitive assets.