The fact-checkers enlisted by Facebook to help clear the site of “fake news” say the social media giant’s refusal to share information is hurting their efforts. In December, Facebook promised to address the spread of misinformation on its platform, in part by working with outside fact-checking groups. But because the company has declined to share any internal data from the project, the fact-checkers say they have no way of determining whether the “disputed” tags they’re affixing to “fake news” articles slow — or perhaps even accelerate — the stories’ spread.