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Shares of Facebook Inc. fell more than 2% in early trading today after the two founders of Instagram left the social networking giant under unexplained circumstances.

The departure of Kevin Systrom and Mike Krieger may have been the result of friction with Chief Executive Officer Mark Zuckerberg over how Facebook’s fastest-growing revenue generator is run.

“Our sense is the duo may have wanted to run Instagram more independently than their parent company wanted,” said Scott Kessler, an analyst at research firm CFRA. “We think the departures are a notable negative for Facebook.”

A Bloomberg report said Systrom and Krieger had been frustrated by a rise in the day-to-day involvement of Zuckerberg, who has become more reliant on Instagram in planning for Facebook’s future.

Zuckerberg described the two as “extraordinary product leaders” and said he wished them all the best and was looking forward to seeing what they build next.

Facebook’s shares were down 2.4% at $161.51 in early trading, knocking more than $11 billion off the stock’s market value. Facebook’s shares are currently down about 6% this year, following six years of stellar gains.

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