An English-language social media propaganda effort that previously criticized Hong Kong protesters and other foes of the Chinese government and has been linked to China has taken the rare step of going after private companies in a strategic industry, researchers said Tuesday. Accounts on Twitter and Facebook have been posing as Texans to attack a rare earths processing facility in that state being built by Lynas Rare Earths Ltd., according to the cybersecurity firm Mandiant. Other accounts in the network in June criticized Canadian rare earths miner Appia Rare Earths & Uranium Corp., which had just announced a new mining...