Some girls gain thousands of followers posting pictures of "thigh gaps" and "bikini bridges," as well as underweight celebrities and thinspirational quotes like model Kate Moss's mantra: "nothing tastes as good as skinny feels."
"It just provides a lot of positivity for them, just in a very maladaptive way," says Edward Selby, of the more visual outlet that sites like Instagram provide.
An assistant professor of clinical psychology at Rutgers University in New Jersey, Selby is the director of a lab there that studies what makes people more likely to develop anorexia (self-starvation), bulimia (binge-eating and purging) and other eating disorders.
About one in 20 young women in Canada has an eating disorder, according to the Toronto-based National Initiative for Eating Disorders.
And people suffering from these diseases often feel good after exercising, purging, swallowing a laxative or doing other things that contribute to their illness, Selby says.
They get caught in a "cyclic feedback loop," with the positive emotions pushing them to engage more in these risky behaviours.
Online pro-anorexia and bulimia communities simply add to that loop by celebrating a person's unhealthy achievements, he says.
The #thinspo world
"Finally under 130! Woohoo!" writes one user with a photo of her feet on a scale. "Yay congrats," reads a response.
Another girl posts a screen grab from an app claiming that she's been fasting for more than a day. It receives 32 likes and a "great job" among the comments.
Some also leverage likes, retweets and comments to set rigid rules about eating and exercise. "Name a food and I won't eat it for two weeks," reads one user's Instagram photo. Another user posts an intricate workout list; for each share, she'll do one set of those exercises.
Not all accounts appearing to promote pro-eating disorder content identify as such, of course. Some post disclaimers denying the association.
Antonia Eriksson, a 20-year-old student living in Linköping, Sweden, perused some of these "thinspirational" accounts nearly three years ago when she was struggling with an eating disorder.
"They're really dangerous," she says. Eriksson is now in recovery from anorexia, and runs an Instagram account and blog focused on fitness and healthy eating. But back then, she was easily triggered into unhealthy behaviour by those images.
"It would help me in my eating disorder, like in the most negative way... It would keep me sick," she says.
When she was hospitalized for anorexia in September 2012, she deleted all those accounts from her Instagram feed.
But while she managed to completely break from viewing all this the pro-eating disorder content, she says it can be easy for people to "get stuck" in the #thinspo world.
More than 100,000 visitors to one pro-ana site
Instagram isn't the first online space on which the pro-anorexia and pro-bulimia community converged. They started springing up as websites and on LiveJournal, an online diary platform launched in 1999.
Dr. Rebecka Peebles, co-director of the Eating Disorder Assessment and Treatment Program at The Children's Hospital of Philadelphia, has been studying pro-anorexia sites for more than a decade.
In 2010, she published an analysis of what's found on more than 200 of these sites — one of which, she says, has more than 100,000 visitors.
Most of the websites her team studied showcased thinspirational images or writing, like the so-called thin commandments, to encourage weight loss and disordered eating. While most offered dieting and fasting advice, nearly half offered tips and tricks specific to disordered eating.
Those types of websites still exist, but the community has since expanded onto social media. Tumblr, Twitter, Instagram and YouTube all have pro-anorexia and pro-bulimia content that is "very, very similar" to the sites Peebles studied.
In 2012, Instagram attempted to curb the community's growth by banning #thinspiration, #probulimia, #proanorexia and other hashtags that glorify self-harm. It also threatened to disable those accounts. But people simply tweaked their hashtag use, relying on more neutral ones (#anorexia, #bulimia, #anagirl) or ones with altered spelling (#thinspooooo, #thygap).
Now, a quick warning about graphic content pops up if someone clicks onto a troublesome hashtag, and users can opt to view the #thinspooooo or be re-directed to the National Eating Disorders Association.
Tumblr, Facebook and Pinterest have also attempted to circumvent users' access to material that promotes self-harm.
#ANAwarriors promote recovery
Peebles has "mixed feelings" about shutting down the sites because much of their content is similar to what shoppers can see on the covers of tabloids at a grocery store checkout.
Additionally, not all the content is inherently negative, she says. Her study found that nearly 40 per cent of the sites included pro-recovery information. Many of the troubling Instagram posts also include pro-recovery tags: #EDrecovery, #ANAwarrior, #BeatANA.
She believes that reflects the nature of the disease. "Part of you wants to get better, and part of you wants to stay sick."
Eriksson was once an #ANAwarrior. She started an Instagram account, which has since grown to nearly 40,000 followers, the day before she was hospitalized to document her six-week in-hospital treatment and recovery.
What she calls her Instagram family helped motivate her recovery. "I wanted to show them that it was possible," she says. "So I just kept fighting it."
As she got stronger and healthier, she changed her account name to eatmoveimprove and now posts pictures of #fitspo or fitness inspiration: fruit-filled oatmeal bowls, selfies with flexed, toned muscle and inspirational quotes.
#Fitspo and orthorexia
Ericsson's and other fitspo accounts are often criticized for promoting another rigid standard of beauty. She says she is often asked about orthorexia, an obsession with healthy eating, but refutes accusations that she's crossed over to the opposite end of the eating disorder spectrum.
She preaches balance: working out for the love of it and nourishing your body to give it energy.
She allows herself to give in to treat cravings, with the occasional Ben & Jerry's ice-cream or slice of pizza.
She acknowledges having bad days and openly discusses them on her blog.
"Today, I deal with my feelings," she says. "Food and anxiety are not connected today. Workouts and anxiety are not connected today."
Selby says health-care professionals can be concerned about recovering anorexics developing orthorexia.
Many with the disease view it as "kind of a friend" he says, and doctors haven't come up with a great answer for how to help them replace it as they move closer toward a healthy weight. He believes it's important to help patients apply the skills that made them successful at weight loss to a healthier, more productive activity.
'Walking wounded' left undiagnosed
Peebles is more concerned about the large eating disordered population that doctors aren't even working with.
One of her studies found that the majority of people frequenting pro-anorexia and pro-bulimia sites had a healthy body mass index (a way doctors measure a person's body fat by using height and weight). Out of more than 1,200 people surveyed, 54 per cent fell within a healthy BMI range. About 21 per cent were overweight or obese.
Despite not being underweight, these people "scored out of the stratosphere" for disordered eating behaviours, says Peebles. Nearly three-quarters of the respondents had purged, binged or used a laxative to help them lose weight.
This means physicians are missing identifying those with eating disorders among seemingly normal, overweight and obese populations, she says.
"Who are the walking wounded with these illnesses that ... are all among us and we're not identifying and helping them?" she asks. "If we were helping them effectively, I think the websites would be a lot less intriguing to them."