People who interact with close friends on Facebook are more likely to eat junk food when they log off, according to US research.
It appears communicating with our friends gives us a confidence boost, which then weakens our self-control so that we're more likely to reach for unhealthy food.
Marketing researchers from Columbia and Pittsburgh universities studied how people's online behaviour affects their off-line habits and found using social media can have an impact on what we eat and how we spend our money.
"Using online social networks can have a positive effect on self-esteem and well-being. However, these increased feelings of self-worth can have a detrimental effect on behavior," wrote Dr Keith Wilcox from Columbia University and Dr Andrew Stephen from the University of Pittsburgh in the Journal of Consumer Research.
"Because consumers care about the image they present to close friends, social network use enhances self-esteem in users who are focused on close friends while browsing their social network. This momentary increase in self-esteem leads them to display less self-control after browsing a social network."
Fifty men and 50 women who use Facebook were involved in the five-part study that used surveys, self-esteem tests and observation to see how the sites influenced behaviour.
People who spent their Facebook time interacting with good friends appeared to lose self-control, while those who focused on acquaintances as well did not have the same response.
People who spent more time on Facebook had a higher body mass index and were more likely to binge eat. They also tended to have higher credit card debt.
But Dr Lauren Rosewarne, a social media expert from the University of Melbourne, told ninemsn that previous research suggests social media negatively affects self-esteem.
"This research suggests when we feel good we eat or shop, but other research indicates it's a self-consoling way to feel better about yourself," she said.
"But we are both getting to the same page at the end of it, it's resulting in behaviour with diminished self control."
Dr Rosewarne said most people only post positive achievements on Facebook, which can cause their "friends" to feel like they have not achieved as much.
"If you log on and think everyone is living a fantastic life and yours is unsatisfactory, you might think, 'What can I do to remedy that?'" she said.
"That tends to result in detrimental behaviours such as impulse purchases."
Dr Rosewarne suggests people monitor their behaviour before, during and after Facebook use.
"Ask yourself, 'What end is this serving for me?'" she suggested.
"If engaging in that behaviour makes your moods enhanced or diminished, then that's a really dangerous cycle to get into. It raises the question of what happens when you are away from it?"