(CNN) -- It's an image that tugs at the heartstrings. A smiling 7-year-old girl poses in her cheerleading uniform, circled by a ring of pompons, her bald head a telltale sign of her chemotherapy treatments.
The photo hit Facebook last year and popped up all over with messages of support. "Like" to show this little girl you care. "Share" to tell her she's beautiful. Pray for her to beat cancer.
But here's the truth. The photo was nearly six years old. And neither the girl, nor her parents -- who never posted it to Facebook -- had any idea it was being used that way.
Welcome to the world of Facebook "like farming."
Those waves of saccharin-sweet posts that sometimes fill your news feed may seem harmless. But all too often, they're being used for nefarious purposes. At best, a complete stranger may be using the photos to stroke their own ego. At worst, experts say, scammers and spammers are using Facebook, often against the site's rules, to make some easy cash.
And they're wiling to play on the good intentions of Facebook users to do it.
"The average user doesn't know any better," said Tim Senft, founder of Facecrooks.com, a website that monitors scams and other illegal or unethical behavior on Facebook. "I think their common sense tells them it's not true, but in the back of their minds, they think 'What if it is true? What does it hurt if I press like?' or whatever."
What does it hurt?
"I was first shocked," said Amanda Rieth of Northampton, Pennsylvania, whose daughter was the subject of that photo. "And then infuriated."
After being notified by a friend who recognized the girl in a Facebook post, Rieth tracked the image back to a link she'd posted to her Photobucket account in a community forum in 2009, two years after it was taken.
Her daughter, who was diagnosed with Stage IV neuroblastoma in early 2007, has been featured in local news segments for her fundraising efforts to fight cancer through Alex's Lemonade Stand. But her mom said she was always part of the decision and was happy to help publicize the fight.
"This? This was entirely different and entirely out of our control," Rieth said. "That's the most gut-wrenching part: the total lack of control."
Hurting the people featured in the posts, and their families, isn't the only risk of sharing such content. Sometimes, a single click can help people who are up to no good.
Often, Senft said, Facebook pages are created with the sole purpose of spreading viral content that will get lots of likes and shares.
Once the page creators have piled up hundreds of thousands of likes and shares, they'll strip the page and promote something else, like products that they get a commission for selling. Or, they may turn around and sell the page through black-market websites to someone who does the same.
It's a way to trick Facebook's algorithm, which is designed to give more value to popular pages than the ones, like scams and spam, that pop up overnight.
"The more likes and shares and comments and that sort of thing you have, the more likely it is to be seen by other people," Senft said. "If they're looking to sell the page in a black-hat forum somewhere, that's what the value of the page is."
It gets worse
Sometimes, the threat is more direct.
The "new" page may be used to spread malware -- software that attacks the user's computer -- or for phishing, the act of trying to gather credit card numbers, passwords or other personal information through links to phony giveaways or contests.
Simply liking a post, or the page itself, can't spread a virus or phish a user. But malicious Facebook apps can, as can external links that page owners may choose to share to their followers.
If the page owner has access to Facebook's developer tools, they can collect data on the people who like the page. Personal information like gender, location and age can be used to target more personalized attacks.
The kind of posts used run a gamut from cute to tasteless, from manipulative to misleading.
Rieth said she still finds her daughter's photo on Facebook from time to time, even though Facebook eventually deleted the original after she and others reported it.
On the most recent page she found, the picture appears in a feed alongside posts such as "Who loves French fries? Like & share if you do" and multiple images encouraging people to like and share if they love Jesus.
There's an image of a premature baby, pictures of military troops cuddling puppies and an image of a young boy pouring water on a man's cigarette with the text "Sorry papa ... I need you."
"It's anything that's going to kind of tug at the heartstrings: the sick kids, the animal abuse, acting like it's some kind of pet shelter," Senft said. "That's the bad part with the scammers. They hit people where they're vulnerable, play on their emotions."
What to do
Because of Facebook's sheer size, he said it sometimes takes lots of reports for the site to delete an offensive or misleading image, or shut down the page it came from. The best approach, Senft said, is to think before sharing.
"If it sounds too good to be true, don't click on it," he said. "If it's something that's obviously geared toward tugging on the heartstrings, check it out first."
Facebook said it continues to work to make sure high-quality content surfaces for users and low-quality posts don't. That includes trying to diminish the reach of posts that appear to be "like farming" attempts.
"People have told us they associate requests to like or share a post with lower quality content, and receiving that type of feedback helps us adjust our systems to get better at showing more high quality posts," a Facebook spokesperson said via e-mail.
"If you see a post that's low quality and seems to be focused only on gaining traffic, hover over the top-right corner of the post and click the arrow to report it."
Facebook uses "automated and manual methods to swiftly remove links and pages that violate our policies," the spokesperson said. "We're always making improvements to our detection and blocking systems to stay ahead of threats."
Today, Rieth's daughter is 13 -- an eighth-grader who has shown no signs of her cancer since September 2007.
But her mom compares that cheerleading photo to the mythical hydra, a monster with many heads that sprouts two more each time one is cut off. Based just on the images she's found and reported, the photo has been liked and shared on Facebook hundreds of thousands of times.
A search Monday also found it popping up on Pinterest, as well as one site where it was wrongly used alongside a 2010 article about actor Jackie Chan helping a girl with leukemia find a bone-marrow donor.
"What makes me truly angry, though, is knowing that they're using it as an insidious way to make money," Rieth said. "That's not what her survival is about to us."
For this article, CNN sent a Facebook message to the owner of the last page where Rieth found the photo.
When asked whether he planned to sell his page, the owner replied with two words: