Facebook has been fined 110m euros (£95m) by the EU for providing "incorrect or misleading" information during its purchase of messaging service WhatsApp in 2014.
The European Commission said Facebook had said it could not automatically match user accounts on its own platform and WhatsApp.
But two years later it launched a service that did just that.
Facebook said the errors it had made were not intentional.
In a statement, the Commission said: "The Commission has found that, contrary to Facebook's statements in the 2014 merger review process, the technical possibility of automatically matching Facebook and WhatsApp users' identities already existed in 2014, and that Facebook staff were aware of such a possibility."
However, it added that the fine would not reverse its decision to clear the $19bn purchase of WhatsApp and was unrelated to separate investigations into data protection issues.
EU Competition Commissioner Margrethe Vestager 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," she added.
Commission rules suggest the social network could have been fined up to 1% of its turnover - a figure that would have been at least twice the amount it has been told to pay.
"The Commission must be able to take decisions about mergers' effects on competition in full knowledge of accurate facts," said Ms Vestager.
In statement, Facebook 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."
Richard Craig, an IT, telecoms and competition expert from law firm Taylor Wessing, said the fine showed that companies had to be more open with regulators during mergers and acquisitions.
Mr Craig said that regulators were overcoming their usual reluctance to consider privacy when scrutinising competition cases.
"The direction of travel is clear that both competition and data protection regulators alike will be vigilant in ensuring that those with access to big data do not utilise it in a way that harms competitors or consumers."
The WhatsApp-Facebook merger has proved troubling elsewhere too.
Last week, Italian anti-trust regulators imposed a 3m-euro fine on WhatsApp for making users agree to share personal data with Facebook.
In addition, French data protection regulators also fined Facebook 150,000 euros for breaking rules on data sharing and user tracking.