Social media sites designed to help people connect are actually causing them to feel more alone, say psychologists.
The more time young adults spend on platforms such as Facebook, Twitter, and Pintrest, the more likely they are to feel cut off from the rest of society, a study has found.
More than two hours of social media use a day doubled the chances of a person experiencing social isolation.
Higher numbers of visits to social media sites have a negative effect as well as the amount of time spent online, the US research shows.
Study participants who visited various sites 58 or more times per week were three times more at risk of isolation than those visiting less than nine times per week.
Lead scientist Professor Brian Primack, from the University of Pittsburgh School of Medicine, said: "This is an important issue to study because mental health problems and social isolation are at epidemic levels among young adults.
"We are inherently social creatures, but modern life tends to compartmentalise us instead of bringing us together.
"While it may seem that social media presents opportunities to fill that social void, I think this study suggests that it may not be the solution people were hoping for."
The team questioned 1,787 adults aged 19 to 32 about their use of the 11 most popular social media platforms at the time the research was conducted in 2014: Facebook, YouTube, Twitter, Google Plus, Instagram, Snapchat, Reddit, Tumblr, Pintrest, Vine and LinkedIn.
Each person was assessed for self-perceived social isolation using a standard technique called the Patient-Reported Outcomes Measurement Information System (Promis) that provides scores for a wide range of measurements.
The link with isolation was found even after taking account of social and demographic factors that might have influenced the results.
Co-author Elizabeth Miller, professor of paediatrics at the University of Pittsburgh, said: "We do not yet know which came first - the social media use or the perceived social isolation.
"It's possible that young adults who initially felt socially isolated turned to social media. Or it could be that their increased use of social media somehow led to feeling isolated from the real world.
"It also could be a combination of both. But even if the social isolation came first, it did not seem to be alleviated by spending time online, even in purportedly social situations."
The scientists have several theories to explain the findings, published in the American Journal of Preventive Medicine.
One is that social media use displaces more authentic experiences because the more time a person spends online, the less time is left for real-world interactions.
In addition, certain aspects of social media may encourage feelings of exclusion, such as seeing photos of friends enjoying an event to which you have not been invited.
Also, exposure to idealised representations of other people's lives may elicit feelings of envy and promote the belief that your life is disappointing and dull in comparison, the researchers believe.
Prof Primack added: "People interact with each other over social media in many different ways.
"In a large population-based study such as this, we report overall tendencies that may or may not apply to each individual.
"I don't doubt that some people using certain platforms in specific ways may find comfort and social connectedness via social media relationships.
"However, the results of this study simply remind us that, on the whole, use of social media tends to be associated with increased social isolation and not decreased social isolation."