Belgium’s federal police issued a public warning on Wednesday afternoon: Facebook’s (fb) reactions—the emoticons of laughter, amazement, anger, sadness and love that have joined the old “thumbs-up”—are actually there to erode your privacy.
Facebook has already had difficulties in Belgium, with privacy authorities there having forced it to stop tracking non-users with its browser cookies. But this is a new one.
Explaining that Facebook’s users are the product, the Belgian police wrote that the new reaction symbols help the social network to evaluate the effectiveness of its advertising.
Get Data Sheet, Fortune’s technology newsletter.
Limiting the number of reactions to six encourages people to express their emotions more frequently and thereby help Facebook’s algorithms, the post read, adding that expressions of happiness tell Facebook when the time is ripe to show ads.
“If it appears that you are in good spirits, Facebook will infer that you are receptive and will be able to sell advertising space by explaining to the advertisers that they are more likely to get a reaction from you,” the police wrote, adding: “One more reason therefore not to rush to click if you want to protect your privacy.”
Of course, one might argue that if you want to protect your privacy from Facebook, don’t go on Facebook—the whole service is designed to analyze its users’ behavior, in order to figure out how to most effectively present advertising. Then again, perhaps this is a handy tip for those who want to use Facebook but not show all their cards to the almighty algorithm.
For more on Facebook, watch:
Facebook had not responded to a request for comment at the time of writing.