Facebook has clarified its mastectomy photo policy after petitions from The Scar Project and Change.org/David Jay.

Facebook announced today that it will allow photos of mastectomy scars after a New York woman posted a petition that garnered 20,000 signatures on Change.org.

"We have long allowed mastectomy photos to be shared on Facebook, as well as educational and scientific photos of the human body and photos of women breastfeeding," Facebook said in a prepared statement. "We only review or remove photos after they have been reported to us by people who see the images in their News Feeds or otherwise discover them. On occasion, we may remove a photo showing mastectomy scarring either by mistake, as our teams review millions of pieces of content daily, or because a photo has violated our terms for other reasons. As a reminder, our terms stipulate that we generally do not allow nudity, with some exceptions as laid out above and here, consistent with other platforms that have many young users."

Facebook's new written policy was hailed by cancer survivors who were outraged that women were not able to share their photos to support others with the disease.

The new policy was established after Facebook had conversations with Scorchy Barrington, a woman struggling with stage IV breast cancer, and photographer David Jay, founder of the SCAR Project.

Facebook has said, however, that photos with fully exposed breasts, especially those that are unaffected by surgery, will not be allowed on the site, in accordance with other policies that govern the site used by so many young people.

Barrington, who uses a pseudonym, according to Change.org, called Facebook's action a "victory … [for] the countless men and women who have this disease and who are newly diagnosed each year. We want the world to know that breast cancer is not a pink ribbon - it is traumatic, it is life-changing, and it urgently needs a cure.

"For some time now, Facebook's policy regarding mastectomy photos was loosely defined, and offered no real assistance to Facebook users posting images and little guidance to Facebook staff tasked with responding to images that were reported," she said.

Numerous photos were removed from The SCAR Project and the Stupid Dumb Breast Cancer page, according to Barrington.

"From now on, these powerful visual testaments to the real impact of breast cancer and the resilience of breast cancer survivors will be welcomed on Facebook, as they should be."

Photographer Jay said in a statement that The Scar Project would be "closely monitoring" Facebook to ensure the policy is implemented.

In February, Facebook removed a series of pages that joked about rape and pedophilia following a Brazilian petition on Change.org. In Australia, a petition led Facebook to remove a derogatory "Aboriginal Memes" page. And last year, Facebook removed several anti-gay pages after a Change.org petition.

Change. org has been a highly effective medium for social change with petitions from ordinary citizens that reached the national stage, including the Trayvon Martin case.

"Change.org has become a powerful platform to flag issues that many people didn't even know existed," spokesman Megan Lubin told ABCNews.com. "It is truly a force to be reckoned with."