Thousands of Facebook users received an unsettling message two years ago: They were being locked out of the social network because Facebook believed they were robots or using fake names. To get back in, the users had to prove they were real. In fact, Facebook knew most of the users were legitimate. The message was a test designed to help improve Facebook's antifraud measures. In the end, no users lost access permanently. The experiment was the work of Facebook's Data Science team, a group of about three dozen researchers with unique access to one of the world's richest data troves: the movements, musings and emotions of Facebook's 1.3 billion users. The little-known group was thrust into the spotlight this week by reports about a 2012 experiment in which the news feeds of nearly 700,000 Facebook users were manipulated to show more positive or negative posts.