Students marching in Chile on Nov. 20, 2011.

SANTIAGO, Chile —  Geography student and young communist Camilla Vallejo needed only to tweet to marshal thousands of her 320,000 followers, with kitchenware in hand, to the streets in protest of the steep costs of education.

“After 5 months we mobilized, more of the same and continued repression more brutal than ever. Bang pots today at 20.30 hrs!,” the 23-year-old tweeted in early October following a large confrontation with police.

Online retailers have long gathered behavioral metrics about how customers shop, tracking their movements through e-shopping pages and using data to make targeted offers based on user profiles. Retailers in meat-space have had tried to replicate that with frequent shopper offers, store credit cards, and other ways to get shoppers to voluntarily give up data on their behavior, but these efforts have lacked the sort of data capacity provided by anonymous store browsers — at least until now. This holiday season, shopping malls in the U.S. have started collecting data about shoppers by tracking t...