Psychologists from Ediniburgh Napier University found in its online and one-on-one study of 200 students that they more friends they had and more time they invested in the , the more likely they were to experience Facebook-associated anxiety.

"We found it was actually those with the most contacts, those who had invested the most time in the site, who were the ones most likely to be stressed," said .Some 12% of respondents said Facebook made them feel anxious, and these respondents averaged 117 Facebook friends vs. 75 for those who said they were not stressed out by using the site.

"The results threw up a number of paradoxes," Charles said.

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The researchers also found "considerable ambivalence" by users about the benefits of Facebook, which has 500 million users and describes its mission as "Giving people the power to share and make the world more open and connected."