(Yui Mok, AP Images) T oday begins a revolution in how personal data is collected and managed online. The European Union implements its General Data Protection Regulation (GDPR), empowering web users to opt out of having their information gathered by websites, with high penalties for non-compliance. Dominant companies have built business models on surveilling and profiling users from every corner of the internet. Nearly all revenue is derived from it, mostly from delivering valuable targeting information to advertisers. The platforms know how much money you make, what you spend it on, what you watch and listen to, whom you owe money to, and what you’re feeling, on a moment-by-moment basis. GDPR could threaten that strategy. “It will require such a rethinking of the way Facebook and Google work, I don’t know what they will do,” says Jonathan Taplin, author of Move Fast and Break Things , a book that’s critical of the platform economy. But last month, The Wall Street Journal and The New...