Spending time on Facebook can be an unhappy experience, German researchers say, as seeing pictures and posts of other users\' can lead to envy and jealousy. The study, titled \"Envy on Facebook: A hidden threat to users\' life satisfaction?\" was conducted at Humboldt University in Berlin and Darmstadt\'s Technical University, the Los Angeles Times reported Wednesday. In a survey of 357 people -- mostly German university students -- about a third said they felt worse after visiting the social networking site and their \"general dissatisfaction\" with life increased. Holiday photos are the biggest cause of resentment, causing more than half of all feelings of envy, the researchers found. While Facebook allows users to keep up to date with a great number of people, it also produces a \"basis for social comparison and envy on an unprecedented scale,\" the study said. \"We were surprised by how many people have a negative experience from Facebook with envy leaving them feeling lonely, frustrated or angry,\" researcher Hanna Krasnova from Humboldt University said.