A new study by US researchers has proven for the first time the causal connection between social media use and decreased mental health.

Few prior studies have attempted to show that social media use harms users' well-being, and even those that did explore the issue were limited in scope or based on unrealistic situations.

"We set out to do a much more comprehensive, rigorous study that was also more ecologically valid," said Melissa Hunt, lead author of the study and associate director of clinical training in the Psychology Department of the University of Pennsylvania.

Researchers at the university designed the study that focused on Facebook, Snapchat and Instagram, the three most popular social media platforms in the US.

A total of 143 undergraduates at the university participated. They were randomly assigned into two groups: The control group was allowed to maintain their typical social media behavior; the experimental group was asked to limit their time on the three social media platforms to 10 minutes per platform per day.

The participants completed a survey on their mood and well-being at the start of the study. After three weeks, they were tested again for outcomes such as fear of missing out, anxiety, depression and loneliness.

Throughout the study, researchers also collected objective usage data automatically tracked by iPhones for active apps, not those running the background.

As a result, researchers found that the group of limited social media use showed significant reductions in loneliness and depression over three weeks compared to the control group.

"Using less social media than you normally would leads to significant decreases in both depression and loneliness. These effects are particularly pronounced for folks who were more depressed when they came into the study," Hunt said in a press release.

It's a little ironic that reducing the use of social media actually makes people feel less lonely, but the findings make sense when digging a little deeper, she said.

"When you look at other people's lives, particularly on Instagram, it's easy to conclude that everyone else's life is cooler or better than yours," she explained.

Hunt admitted that the study only looked at three major social media platforms and that the results may vary if applied to different age groups.

The study was published in the latest issue of the Journal of Social and Clinical Psychology.