With friends like these, who needs enemies? A new study has found that the more Facebook "friends" you have, the more likely you are to feel stressed out by the social networking site.

Psychologists from Edinburgh Napier University quizzed around 200 students about their use of Facebook and concluded that, for a significant number of users, the negative effects of Facebook outweighed the benefits of staying in touch with friends and family.

Facebook is now the most-visited website in the UK, and has over 500 million users worldwide. If Facebook were a country, it would be the third most populated in the world, behind only China and India.

The researchers found that among their students, those with the most Facebook "friends" were the most likely to be stressed out by the site, 12 per cent saying that it made them feel anxious, 32 per cent saying rejecting "friend" requests had led to feelings of guilt and discomfort, and 10 per cent saying they don't like getting "friend" requests in the first place.

"The results threw up a number of paradoxes," said Dr Kathy Charles, who led the study. "For instance, although there is great pressure to be on Facebook there is also considerable ambivalence among users about its benefits.

"Our data also suggests that there is a significant minority of users who experience considerable Facebook-related anxiety, with only very modest or tenuous rewards," Dr Charles said. "And 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."

Some students also said they were anxious about withdrawing from the site for fear of missing important social information or offending their friends.

"An overwhelming majority of respondents reported that the best thing about Facebook was 'keeping in touch', often without any further explanation," said Dr Charles. "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."

She said other causes of tension included purging unwanted contacts, having to use appropriate etiquette for different types of friends and the pressure to be inventive and entertaining. That's surely something Steven Fry will identify with: he briefly abandoned the rival social networking site Twitter after one of his "followers" accused him of being boring.

Dr Charles added: "The other responses we got in focus groups and one-to-one interviews suggests that the survey figures actually under-represent aspects of stress and anxiety felt by some Facebook users, whether it's through feelings of exclusion, pressure to be entertaining, paranoia or envy of others' lifestyles."

The most common searches on Google starting with "Are Facebook friends . . ." are: "Are Facebook friends really your friends?" and "Are Facebook friends real?". Real or not, it seems having a lot of "friends" on the site is a source of stress for many.

In January, a survey in the US by the Toluna market research firm and the phone firm VTech Communications found many suffering from what some have labelled "digital stress": anxiety brought on by having to be constantly accessible for work (33 per cent) and the apparent need to keep up with the latest technologies (20 per cent). In a New Year promotion, VTech asked punters how they intended to avoid such stresses this year, and guess where they hosted the competition? You guessed it: on their Facebook page.

Jason Stamper is NS technology correspondent and editor of Computer Business Review.