Facebook is spending heavily to avoid a repeat of the Russian interference that played out on its service in 2016. The social network is bringing on thousands of human moderators and advanced artificial intelligence systems to weed out fake accounts and foreign propaganda campaigns. But it may never get the upper hand. Experts say its adversaries are adept at camouflaging themselves and aren’t always detectable by AI. They constantly test Facebook’s countermeasures and then exploit whatever holes they find. Thirty-two fake accounts and pages recently disclosed by Facebook revealed that operatives likely tied to Russia are using the same techniques to sow racial, social and political division ahead of the 2018 midterm elections. Google, Twitter and Reddit have said they are working to prevent misuse of their services too.