A study carried out on behalf of the Wall Street Journal found that "Like" and "Tweet" widgets on websites track users' movements across the internet even when they're not used, with Facebook admitting to sharing data for advertising purposes only when the widget is clicked on.
Twitter, meanwhile, claims to delete the information quickly, although a spokesman said that the company "could in theory use the data to 'surface better content' for users in the future."
The study, carried out by former Google engineer Brian Kennish's new comapny, Disconnect Inc., surveyed the 1,000 most popular sites online according to Google, and found that Facebook collected browsing data from 331 of them, with Twitter collecting data from "about 200." The collection of browsing data is described by their creators as an "unintended side-effect" of the way that the widgets work, with Facebook's CTO Bret Taylor telling the WSJ "We don't use them for tracking and they're not intended for tracking." Well, not yet, at least.
Source: techland.time.com