For young adults, social media may not be so social after all.

Among people in that age group, heavy use of platforms such as Facebook, Snapchat and Instagram was associated with feelings of social isolation, a study finds.

The results surprised study co-author Brian Primack. "It's social media, so aren't people going to be socially connected?" he says. He's director of the Center for Research on Media, Technology and Health at the University of Pittsburgh. And while his team's previous research connecting social media use and depression in young adults wasn't terribly surprising, these new results seemed counterintuitive.

While face-to-face social connectedness is strongly associated with well-being, it's not clear what happens when those interactions happen virtually. To investigate, Primack and his colleagues surveyed 1,787 U.S. adults ages 19 to 32 and asked them about their usage of 11 social media platforms outside of work. The survey also gauged social isolation by asking participants questions such as how often they felt left out. (As will happen in this type of survey, people may have lowballed their estimates of media use.)

It turns out that the people who reported spending the most time on social media — more than two hours a day — had twice the odds of perceived social isolation than those who said they spent a half hour per day or less on those sites. And people who visited social media platforms most frequently, 58 visits per week or more, had more than three times the odds of perceived social isolation than those who visited fewer than nine times per week. The study appeared Monday in the American Journal of Preventive Medicine.

This study can't nail down causation. It could be that when people feel socially isolated, they go online a lot in an attempt to feel less lonely, says Primack. Or it might be that spending a lot of time on social media makes people feel isolated.

"You might watch all these interactions where it seems like everyone else is connecting," he says. That could lead to feeling excluded. The images of other people's seemingly perfect vacations, homes and lives, even though those are not likely to represent reality, can make you feel like you're missing out.

It could be that influence goes both ways — isolation drives social media use and vice versa. Or there might be some unknown factor that is leading to both social isolation and social media use. (The researchers controlled for the major potential influencers including age, gender, relationship status, household income and education.)

Primack says the results shouldn't be interpreted to mean that we should ditch our social media accounts, but there's a lot to dig into to see how best to use them. For example, future research could focus on whether outcomes vary if people are using social media actively or passively, for heated political disagreements or to click "like" on puppy pictures, and to extend their in-person social network or as an end in itself.

"What we know at this point is that we have evidence that replacing your real-world relationships with social media use is detrimental to your well-being," says Holly Shakya, an assistant professor in the division of global public health at the University of California, San Diego, who wasn't involved in the study. "But we need further research to tease out the mechanisms and replicate the findings," she says. This study, she says, is another piece of the puzzle.

Shakya was an author of a study published in January that tracked Facebook use and well-being over time and found the use of the social network was negatively associated with factors including physical health, mental health and life satisfaction. Offline interactions, meantime, had positive effects.

"Where we want to be cautious ... is when the sound of a voice or a cup of coffee with a friend is replaced with 'likes' on a post," she says.

Primack notes that his study was conducted in a specific age group, and shouldn't be generalized to older or younger people. Both social isolation and social media use are very different across the age spectrum; for example, young adults generally have a lot of different opportunities for in-person experiences and so social media use may represent a retreat from that, while older adults are more likely to feel socially isolated and might benefit from having opportunities to connect online.

Katherine Hobson is a freelance health and science writer based in Brooklyn, N.Y. She's on Twitter:@katherinehobson.

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