LONDON—European Union and British regulators opened dual antitrust investigations Friday into whether Facebook distorts competition in the classified advertising market by using data to compete unfairly against rival services. The coordinated effort represents an escalation by European regulators in their battle to rein in the dominance of big tech companies. The focus of the investigations highlights a longstanding concern that the data they collect from people or businesses using their platforms are used to get an advantage over competitors, which could include those same businesses. “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 Margrethe Vestager, the European Commission’s executive vice president in charge of competition policy. “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 …