The emotions of vulnerable teenagers can be analysed by Facebook and shared with advertisers, warned a secret document.
It suggested the social media giant is monitoring the activity of youngsters to find out how they are feeling.
The leaked internal research detailed how the mood of users as young as 14 can be ascertained by key words.
These could include being concerned with "looking good" or "losing weight". Using complex algorithms, Facebook can work out if teenagers feel emotions such as being "overwhelmed, defeated, stressed, anxious, nervous, stupid, silly, useless or a failure".
This detailed data, which Facebook calls "sentiment analysis", could then be used by advertisers to target young, potentially more impressionable users.
The 23-page document marked ‘Confidential: Internal Only’ and dated 2017 was obtained by The Australian newspaper.
It was said to have been compiled by two executives in Australia and to focus on youngsters there and in New Zealand.
The document reportedly included information on when young users are most likely to feel certain emotions and included analysis of those as young as 14.
When contacted by the Daily Mail, Facebook admitted an "oversight" with the research which is being investigated.
But it denied offering tools to target people based on their emotional state. It said the research was shared with an advertiser but featured "anonymous" data and was not used to target adverts.
The document claimed Facebook can also understand how emotions are posted at different times of the week.
"Anticipatory emotions are more likely to be expressed early in the week, while reflective emotions increase on the weekend," it reportedly stated.
"Monday-Thursday is about building confidence; the weekend is for broadcasting achievements". This is not the first time Facebook has caused controversy by monitoring emotions.
In 2012, it ran an experiment on some users, changing which posts appeared in their news feed to find out if they became sad by seeing more numerous negative posts.
The results showed this did happen but critics claimed Facebook was engaging in social engineering for commercial benefit.
The site states that children under 13 should not use it. But research in 2014 found that 52 percent of eight to 16-year-olds admitted they had ignored the official minimum age limit when they joined.