The growing dominance of Google and Facebook in the online ad market has loomed over many conversations at the Cannes Lions advertising festival in France this week, according to a report in the Wall Street Journal. AOL CEO Tim Armstrong said the topic has come up in nearly every meeting he has taken. “It comes up with publishers, with advertising agencies, with marketers. There’s a fear of a duopoly overall,” he said in an interview Tuesday in Cannes. In fact, last year, Google and Facebook together commanded 43% of global ad revenue online, according to an estimate from research firm eMarketer. Armstrong said the concerns stem from so-called walled garden strategies that Facebook and Google are pursuing which are increasingly restricting third-party companies from operating on their networks and from accessing their data. Facebook and Google are, of course, directing marketers to use their own products. In contrast, AOL plans to employ a more open ad tech strategy, according to Armstrong.