LONDON — Lie detectors have a deservedly flaky reputation. And in the era of “fake news,” “untrustworthiness detectors” may be no better. Facebook is now fighting fake news and other fake, false or flaky content, partly in collaboration with traditional media organizations, by trying to detect, measure and then reduce untrustworthy content. This can be difficult. Fact-checking is often laborious, expensive and contentious. Attempts to take down misleading content often have limited success — in part because fake, false and flaky content is more likely to be sensational and to be repeated and recirculated than boring factual content.