By not providing enough information to its users, Facebook collected the data without getting their consent and violated data protection regulations, the agency alleges.
Spain's AEPD conducted the probe alongside others in Europe, and says that Facebook didn't sufficiently inform its users about how it would use the data.
"The social network uses specifically protected data for advertising, among other purposes, without obtaining users' express consent as data protection law demands, a serious infringement".
That Facebook is causing ripples when it comes to privacy will come as little surprise to anyone.
The company has been tracking both users and non-users of the service through the Like button across the web without informing them about this sort of tracking, nor about what it plans to do with the data.
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Finally, the AEPD also noticed that Facebook has not been completely purging the data about users who had already deleted their accounts and that Facebook was making use of accounts' data that have been deleted for more than 17 months.
Facebook has been fined for similar violations in the past by Belgium, which it successfully overturned the ruling, as well as France, the Netherlands, and Germany.
Under EU law, "personal data" means "any information relating to an identified or identifiable natural person", so people's "likes" would qualify as personal data.
Shares of Facebook are now up about 1.2% in late-morning trading, while the stock is a #3 (Hold) on the Zacks Rank. In addition, it had ordered Facebook to stop transfers of personal data to the U.S.