Police in Belgium have warned residents there to avoid using Facebook's reactions feature to ensure they are not targeted by advertisers. 

"Facebook never lets an opportunity to gather more information about us pass," the Belgian police force said in a statement. "The [reactions] icons help not only express your feelings, they also help Facebook assess the effectiveness of ads on your profile." 

"One more reason to not rush to click if you want to protect your privacy," the police statement ended. 

Facebook introduced its reactions feature last February, adding laughter, amazement, anger, sadness and love to the ubiquitous 'like' button. 

The social network had said the reactions would give users "more ways to easily and quickly express how something you see in News Feed makes you feel." Its blog post introducing the feature made no mention of the reactions' advertising potential. 

It's been a rough week for the world's largest social network, which has spent the past days responding to claims that its trending news section is biased against right-wing political perspectives. 

The allegations were first published in an article on tech site Gizmodo, which quoted an anonymous former Facebook staffer as saying they routinely suppressed conservative news. 

Facebook founder and CEO Mark Zuckerberg has denied the allegations but said the company is conducting a thorough investigation.