A little more than two weeks after Facebook announced a news initiative, the social network was on the defensive, responding to a published report that a website co-founded by the company’s top news executive had displayed bias against Elizabeth Warren.

Popular Information, a liberal newsletter, reported on Monday that The 74, a nonprofit education policy website co-founded by Facebook’s top news executive, Campbell Brown, had published several articles attacking Ms. Warren, a senator from Massachusetts and Democratic presidential candidate. One of them called Ms. Warren “the second coming of Karl Marx.”

The newsletter entry was written by Judd Legum, a former editor of the defunct liberal website ThinkProgress and a onetime Democratic political operative. It argued that the articles critical of Ms. Warren that appeared on The 74 were relevant to Facebook’s move into the news business, because Ms. Brown still sits on the site’s board. Mr. Legum also reported that the family foundation of the education secretary, Betsy DeVos, was a donor to The 74.

Ms. Brown responded swiftly to the criticism on Monday, saying in a Twitter thread that her involvement with The 74 had no bearing on Facebook News and that the articles singled out by the newsletter were opinion essays, not news articles.