Weeks after Facebook Inc. disclosed it had been exploited by Russia-backed propagandists, the company was still underestimating its problems, with users and advertisers and, crucially, Washington.

At an October off-the-record conference of tech-industry elites in Hawaii, Facebook’s head of advertising, Rob Goldman, defended Facebook’s response and argued the Russians bought ads to exploit social divisions, not primarily to sway the 2016 U.S. presidential election, say people who heard his remarks.

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