Facebook screenshot

Posing as a 13-year-old called Lucy Greening from London, a Daily Mail reporter wrote Facebook posts such as “feeling skinny makes me happy”, and how the teenager felt she was falling behind her peers.

On the account, “Lucy” searched the social media site for terms such as “too fat”, “how to get thigh gap” and “how to lose weight fast”.

The schoolgirl stumbled upon an image of a naked couple having sex.

The endlessly looping video, known as a gif, was posted on a Facebook page called “Porno Sex Videos +18” even though porn is supposed to be outlawed on the social network.

“Lucy” was also shown numerous posts by users selling diet pills and weight loss supplements on third-party websites.

One product promised users they could lose up to 1.36kg a day, but demanded they restrict their diet to no more than 1 200 calories.

Another said users should consume no more than 900 calories.

The imaginary teenager was presented with an article offering a guide to techniques such as a “zero calorie diet”.

One entry suggested taking up smoking while another encouraged users to make themselves throw up.

Towards the end of the experiment, “Lucy” searched the term “thinspo” a word used to refer to “motivational” pictures of thin people.

Facebook immediately offered a drop-down menu of longer terms she could be looking for including “thinspo pro ana [anorexia]” and “thinspo tips”.

Clicking on these options led to even more potentially harmful content, including a page called “Skinny idol” offering a list of extreme weight loss methods.

A hypnotic video of a young woman throwing up into a toilet played on an endless loop under the caption “pain is beauty”.