"It is illegal for a business to use its market power to engage in anti-competitive conduct in an effort to achieve or maintain a monopoly," Nessel said in a statement.

"Our personal data is the biggest commodity in today’s online economy and, as the chief law enforcement officer of the state, it is my duty to ensure Michigan residents’ personal data doesn’t continue to be pillaged in a monopolist’s quest to control social media and advertising markets," Nessel said.

"A lot of Michiganders use Facebook, which prompted Nessel's entrance into the investigation," attorney general's office Communications and Media Relations Officer Dan Olsen said.