Mark Zuckerberg on Saturday vigorously defended Facebook Inc. against criticism that fake news and misinformation on the social media site may have swayed the outcome of the presidential election, emphasizing his view that Facebook’s role isn’t to be “arbiters of truth.”

Mr. Zuckerberg, Facebook’s chief executive and co-founder, said in a lengthy post that less than 1% of the site’s world-wide content could be classified as fake.

“Overall,...