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LAGUNA BEACH, Calif. — Social media platforms have reached a crucial point where the executives overseeing them need to find ways to anticipate and block bad actors, Instagram co-founder Kevin Systrom said.

Over the last eight years, social-media services like Instagram and Facebook Inc. FB +0.94% have attracted an unparalleled number of users. Instagram has more than a billion users, while Facebook’s monthly user base exceeds 2 billion.

“The reach is so tremendous that it’s almost like the laws of physics no longer apply,” Mr. Systrom said at The Wall Street Journal’s WSJ Tech D.Live conference on Tuesday. “Now that we’re at this scale, what are the implications?”

Problems like the Russian meddling in elections and the Cambridge Analytica crisis earlier this year underscore how important it is for executives to grapple with the downsides of their platforms, said Mr. Systrom, who recently left the company he founded and plans to start another venture with his Instagram co-founder. Doctored photos and videos aren’t convincing yet, but they’re “getting on the margin of real,” he added.

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Last week, the gunman who killed 12 people at a Southern California bar was posting on Facebook and Instagram immediately before and during the massacre.

All these together should put the growing social-media industry on notice, he said. “You start to realize…how important it’s going to be for the future of the world that we police these things well, we take it very seriously and put real resources against solving the problems,” he said.

Mr. Systrom and his Instagram co-founder, Mike Krieger, resigned from the company they founded in September after mounting tension between the pair and Facebook Chief Executive Mark Zuckerberg, the Journal previously reported. Facebook bought Instagram in 2012 for $1 billion.

To read more of this story, which originally ran in The Wall Street Journal, click here.