Facebook spokesman Andy Stone said the company investigated a claim by an outside researcher over suspicious pro-Sanders content, among regular reports it generally receives from the research community. While Facebook actively monitors its platform for political disinformation, not all internal inquiries are escalated to senior officials, as this one was, people familiar with the matter said.

"We investigate each credible claim we receive, just as we did in this instance when an outside researcher contacted us," Mr. Stone said. "To date, we have not been able to substantiate the researcher's claims and we have not been notified by the intelligence community." -WSJ

Twitter, meanwhile, removed 70 pro-Bloomberg accounts for violating their "platform manipulation" policy.