At issue is whether the social-media giant is deploying data it gleans from advertisers to compete against them in classified ads.
The U.K. is also looking at whether Facebook uses ad data in the same way for its online-dating service.
“Facebook collects vast troves of data on the activities of users of its social network and beyond, enabling it to target specific customer groups,” said EU competition chief Margrethe Vestager.
“We will look in detail at whether this data gives Facebook an undue competitive advantage, in particular on the online classified ads sector, where people buy and sell goods every day, and where Facebook also competes with companies from which it collects data.”
Facebook’s stock at last check firmed 0.1% to $326.50. The stock has climbed 16% in the past six months.
Last month, Citigroup analyst Jason Bazinet downgraded Facebook to neutral from buy, keeping his price target at $320.
Also in May, Facebook’s independent oversight board upheld the ban of former President Donald Trump from Facebook and its Instagram platform. But it also ruled that the company shouldn’t impose an "indefinite" suspension.