EU antitrust regulators opened an in-depth investigation on Monday into Facebook's acquisition of US customer service startup Kustomer, concerned that the deal could hurt competition.
The European Commission said it had concerns the proposed deal would reduce competition in the market for customer relationship management software.
It also expressed concern that the acquisition would strengthen Facebook's position in online display advertising by increasing the amount of data available for Facebook to personalise its ads.
Last month, Reuters exclusively reported that the deal would face a full-scale EU investigation.
The world's largest social network announced the deal to scale up its instant messaging app WhatsApp in November last year.
The European Commission, which oversees competition policy in the 27-nation European Union, said it had given itself until Dec. 22 to take a decision.