(Reuters Breakingviews) - Facebook's social contract is coming unstuck. Its users provide massive amounts of data, which the company uses to sell advertising. In return, it provides a free service. The same is true of search engine Google. But while more data makes Google's service better and more accurate, it has tended to make Facebook less entertaining. With questions about how Facebook data was used in political campaigns, that could make users more wary of the trade-off.