Meta has been hit with a €390m fine (£343m) fine by regulators for forcing users to agree to personalised adverts.
The penalty has been handed out by Ireland's Data Protection Commission, which said the social media giant had breached EU privacy rules.
As well as the hefty fine, Meta has also been banned from forcing users to sign up to such ads.
The promotions are based on someone's online activity, allowing advertisers to target those likely to be interested in their products and services.
Meta, formerly known as Facebook, has said it will appeal the fine and the substance of the ruling itself.
"The decisions relate only to which legal basis Meta uses when offering certain advertising," a spokesperson said.
The fine dwarfs a €225m (then £193m) penalty handed out by the Irish watchdog to WhatsApp in 2021, when the company was found to have broken regulations in relation to transparency of data shared with other Meta firms.
Ireland's Data Protection Commission issues such fines in its role as the lead privacy regulator for many of the world's largest technology companies within the EU.
The latest penalty brings the total levied against Meta by the watchdog to €1.3bn (£1.1bn).
Reuters reports that the fine comes after an EU directive last month on how social media firms run advertising.
It related to a change to Facebook and Instagram's terms of service in 2018 following the introduction of new EU privacy regulation, where Meta sought to rely on the so-called "contract" legal basis for how it handled ads.
It had previously relied on the explicit consent of users to the processing of their personal data for targeted ads, and Meta considered acceptance of its updated 2018 terms as an all-clear to maintain such advertising.
A company spokesperson argued that there was "a lack of regulatory clarity on this issue".
The fine comes just weeks after Meta came under scrutiny over the presence of potentially fraudulent adverts on Facebook and Instagram.
Analysis of adverts by consumer group Which? and research charity Demos found hundreds of paid-for promotions showcasing unregulated investment products, including cryptocurrencies and non-fungible tokens.