Last summer, Johannes Baldauf, an anti-hate-speech activist from Berlin, got a request for help from an unexpected place: Facebook Inc. When it came to stemming the spread of extremist messaging, Baldauf was accustomed to seeing the giant social network as part of the problem. Now it was asking him and other activists to act as a kinder, gentler, online version of Quentin Tarantino's Nazi-hunting  Inglourious Basterds  squad.