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Six big U.K. advertisers just took Facebook off their friends list. First Direct Bank, Vodafone, Virgin Media, the U.K.'s Automobile Association, Halifax Bank, and Prudential all took offense when their ads appeared on the British National Party's Facebook group page. Many cited corporate policies prohibiting ads from appearing to support political parties. Virgin Media, more forthrightly, simply needed to "protect its brand." No wonder: The BNP has called for Muslims to be banned from Britain's skies — one of its less extreme positions — and drawn vehement protest rallies. This incident, of course, plays on advertisers' worst fears — and Facebook's.

Advertisers have long worried about the lack of control they have over where their ads appear on the Web. Despite promises, targeting and exclusion algorithms don't work nearly well enough — ever seen travel ads appear next to a story on a fiery airplane crash? Exactly. And on free-for-all social networks, the problem's even worse.

With its roots on politically active college campuses, it's going to be hard for Facebook to find safe spaces for marketers averse to controversy. And any move to crack down on political activism, no matter how distasteful, will just enrage Facebook's outspoken users. Is this why advertisers are paying Facebook's exorbitant rates — to ally themselves with neo-fascist parties? Surely not, they must be thinking.