Experts say the cost of what teens see as an ideal body shape -- but really is for most unattainable -- is self-esteem problems that can lead to eating disorders, depression and even suicide.
On Tumblr, Pinterest and Facebook, "thigh gap" photos abound: close-ups of sometimes unbearably skinny legs published by young girls eager to show off their success -- or bemoan what they see as a failure to whittle away fat.
"My thigh gap is huge," brags a Tumblr user with the handle foster-the-beatles.
Another user, skinnysizezero, cheers her fellow dieters on, saying: "Together we can lose weight. Together we can be skinny."
"Together we can be a size zero with a beautiful thigh gap and flat stomach. Together we can be happy and finally say that we love our bodies," her post pledges.
Another poster, elleskyyy, said she felt better when she "realized I'm getting a thigh gap."
Meanwhile, a user called "starving for perfection" complained about her "mediocre/nonexistent thigh gap" and flagellated herself for her "fatfatfatfat."
Experts say the obsession with leg shape is not new, but has been dramatically amplified by social media websites and their 24/7 influence on the lives of American teens.
The fan Twitter account Cara's Thigh Gap, is dedicated to the extreme slenderness of British model Cara Delevingne, while dozens of Facebook pages and websites propose diets and exercise regimes to achieve the almighty gap.
But clinical psychologist Barbara Greenberg warned that for most women, the "thigh gap" is a pipe dream, even via extreme dieting and exercise.
"Most women are not built that way to have that space between their thighs," she said.
"It is a matter of bone structure," she explained, which "the majority of women do not have."
For teenagers, adopting what Greenberg called an "unrealistic obsession" can be dangerous -- increasing pressures that can lead to depression, even suicidal behavior, as well as to severe eating disorders, which can cause lasting brain and bone damage.
Indeed, starvation diets -- and self-loathing -- are a common theme on the "thigh gap" pages of young girls.
"Yesterday i had 380 calories but then i ate candys so much that my calorie number switch to ca. 650?. faaaaaaaaaaaaaat," writes Anastasia, a young German girl, on Tumblr, who prays: "Please God let me be skinny."
The World Health Organization recommends a daily intake of around 2,500 calories to support the energy needs of a growing teenage girl.
'Thin ideal' spread in media
Shannon Snapp, a sociologist at the University of Arizona, blames magazines, movies and television for spreading the "thin ideal," and urges consumers to stop buying into it.
"That message is internalized by young women and girls: if you want to be successful, if you want to be liked, this is the way you should look," Snapp said -- "thin everywhere except for their breasts."
"Teenage girls are probably the most likely to be feeling the pressure to look that particular way, because they are going through puberty: for the first time, they are compared to adult women," she added.
Likewise, San Jose State University sociologist Natalie Boero said the skinny-obsessed "are looking for social acceptance and to fit in."
"Young women are aware that in a sexist and sizeist culture, their bodies are their currency, and they are looking to increase their perceived social value," she added.
That's not how the teenagers themselves always see it, however.
One girl, "mannddda," declares on Tumblr: "I hate when people tell me I'm dumb for wanting a thigh gap and to be skinny. It's not for anyone else but ME. I wanna look in the mirror and be happy for MYSELF & NO ONE ELSE."
But sociologists say there is an unmistakable trend linking body size with social status.
Abigail Saguy, a body image expert at the University of California, Los Angeles, told AFP: "Attaining thinness is a way of signaling elite social status."
But, worse than that, "fatness not only connotates low social status, but it may predict low social status."
"Studies show that heavier girls and women are less likely to get hired and when they are hired, they are paid less," Saguy said, adding that larger women are also less likely to marry.
However, a counter-movement against the "thigh gap" is building, with girls also taking to social media to mock the obsession.
One YouTube video, "5 Ways to Fake a Thigh Gap," posted by "tadelesmith," suggests, for example, that girls who want a gap between their thighs should move their legs apart.
And on Twitter, Common White Girl declares herself relieved that her thighs touch, saying: "Not having a thigh gap saved my phone from falling in the toilet."