Over the years, we’ve been warned about various dangers lurking on the internet: computer viruses, phishing scams, pornography, illegal downloading. But there’s another insidious side effect of the internet we rarely talk about: envy.It turns out, as many as one in three people who use Facebook and similar social network sites experience negative feelings such as envy and jealousy after spending time on the site, according to a new study conducted jointly by two German universities.Researchers from the Institute of Information Systems at Berlin's Humboldt University and from Darmstadt's Technical University conducted two surveys involving 600 people. The first study looked at the scale, scope and nature of envy incidents triggered by the world’s largest social network, the second at how envy was linked to passive use of the site and life satisfaction.The study found that checking out our friends’ vacations, love lives and work successes on Facebook can provoke depression, anxiety and envy.The researchers say 30 percent of the study’s participants felt worse and more dissatisfied with their lives after visiting the social networking site. Another 36 percent said they were at least “sometimes” frustrated after visiting Facebook."We were surprised by how many people have a negative experience from Facebook with envy leaving them feeling lonely, frustrated or angry," said Hanna Krasnova, a researcher from the Institute of Information Systems at Berlin's Humboldt University.What seemed to do the most emotional damage in those users was seeing positive posts and photos of Facebook friends who were smiling and looking happy. The researchers also found that those who browsed the social-network site but did not contribute tend to be the ones who are the most unhappy and dissatisfied with their lives."Passive following triggers invidious emotions, with users mainly envying happiness of others, the way others spend their vacations and socialise," the researchers wrote in the report "Envy on Facebook: A Hidden Threat to Users' Life Satisfaction?" released yesterday.The researchers found vacation photos were the biggest cause of resentment with more than half of envy incidents triggered by holiday snaps on Facebook. Social interaction was the second most common cause of envy as users could compare how many birthday greetings they received to those of their Facebook friends and how many "likes" or comments were made on photos and postings.The study found people in their mid-30s were most likely to envy family happiness while women were more likely to envy physical attractiveness.While the participants in both studies were German, the researchers said they expected the findings to hold internationally as envy is a universal feeling.This isn’t the first study on “Facebook Envy”. A study published in the December 2012 edition of the Cyberpsychology, Behavior and Social Networking Journal found that the more time college students spent on Facebook, the worse they felt about their own lives.And a Stanford University study published in 2011 in the Personality and Social Psychology Bulletin found that people with “Facebook Envy” routinely overestimate the happiness of others.But could these negative feelings one day impact Facebook usage?"From our observations some of these people will then leave Facebook or at least reduce their use of the site," Ms Krasnova said. "From a provider's perspective, our findings signal that users frequently perceive Facebook as a stressful environment, which may, in the long-run, endanger platform sustainability," the German researchers concluded.