Facebook has been fined €110m (£94m) by the EU for misleading officials over its takeover of WhatsApp in 2014.
The European Commission says Facebook knew it would be able to share data between Facebook and WhatsApp users' accounts, despite claims to the contrary.
When Facebook notified the Commission over the acquisition of WhatsApp in 2014, it said it would be unable to automatically link people's accounts.
However, in August 2016, WhatsApp announced updates to its terms of service, including the possibility of linking WhatsApp users' phone numbers with their Facebook identities.
The move prompted the Commission to launch an investigation.
The Commission found that, contrary to Facebook's statements in 2014 during the merger review process, the possibility of automatically matching Facebook and WhatsApp identities already existed, and that staff were aware of this.
Moreover, the Commission says Facebook was also aware of the importance of this factor in the Commission's assessment of the merger.
Facebook was found guilty of two separate infringements by providing incorrect and misleading information in the merger notification form, and in its reply to a Commission request for information.
Commissioner Margrethe Vestager, who is in charge of competition policy, said: "Today's decision sends a clear signal to companies that they must comply with all aspects of EU merger rules, including the obligation to provide correct information.
“And it imposes a proportionate and deterrent fine on Facebook. The Commission must be able to take decisions about mergers' effects on competition in full knowledge of accurate facts."
However, the Commission said had Facebook provided the information it would not have prevented the merger being approved, because of the level of competition in the market.
A Facebook spokesperson said: “We've acted in good faith since our very first interactions with the Commission and we've sought to provide accurate information at every turn.
"The errors we made in our 2014 filings were not intentional and the Commission has confirmed that they did not impact the outcome of the merger review. Today's announcement brings this matter to a close.”