The saying goes that "You can't have too many friends," but that might not be true in the world of Facebook, according to a new study by Scottish researchers.

Psychologists from Ediniburgh Napier University found in its online and one-on-one study of 200 students that they more Facebook friends they had and more time they invested in the social network site, 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 Kathy Charles, who led the study.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."

Among the predictable stress-inducing aspects of Facebook is rejecting a friend request, which 32% of respondents said made them feel guilty or uncomfortable, and the feeling pressure to be entertaining without offending certain groups off friends. Also, while participating in Facebook can be stressful, so can the decision to bail on it for fear of missing out on things or offending people, the researchers found.

"Like gambling, Facebook keeps users in a neurotic limbo, not knowing whether they should hang on in there just in case they miss out on something good," said Charles in a statement.

Not surprisingly, the rise of social networking has launched something of a research frenzy on the topic. Among other research efforts in this field, University of Colorado Denver Business School studied why people unfriend each other on Facebook, and learned that the #1 reason was getting too many useless posts. Other studies have focused on the impact using social network sites at work have on worker productivity.