Rob Scott lashed out at an unknown man he heard make an unkind comment about his son’s Down syndrome, and the resulting video went viral. ( YouTube)

When Megan Harris decided to vent her frustration on Facebook, she had no idea where it would go. She just knew she was bothered by a saleswoman who she felt had insulted her teen daughter.

You entered and told my daughter she needed to wear SPANX if she wanted to wear this dress, Harris wrote in her post, addressing the nameless sales associate from a Dillard’s department store in Kansas. I wish I had told you how many girls suffer from poor self image and telling them they need something to make them perfect can be very damaging.

Can you guess what happened next? The post was shared by dozens of her friends, then hundreds of other people and ultimately nearly 100,000 Facebook users. In a matter of days, Harris and her daughter, Lexi, went viral — and so did that anonymous Dillard’s saleswoman.

For as long as it’s been around, the Internet has given us a forum to rage at whoever is offending us: Osama bin Laden, Anne Hathaway, Martin Shkreli, Donald Trump, Jonathan Franzen, Nickelback, that dentist who shot a lion, that publicist who made a bad joke about AIDS on Twitter. And although the targets of our ire have been getting smaller and less famous — like the Indiana pizza shop owners whose vow to never cater a gay wedding triggered crank calls and boycott threats from across the country — today’s most viral villains are often virtual nobodies.

Thanks to social media, we can publicly scold the commuter who cut us off in traffic, enlighten the ignorant customer who sniped at us in the checkout line, pillory the neighbor who made a cruel remark.

These are the latest targets of those impassioned “open letters” filling up Facebook. Frequently, they decry acts that are perhaps less criminal than simply unkind. But they spring to life as everyday parables — dramatic enough to be compelling but ordinary enough to be relatable — and they come with a “share” button, so: click.

Megan Harris posted an open letter to a Dillard's saleswoman whom she felt had insulted her daughter. The reaction went viral on Facebook. (Facebook/Courtesy of Megan Naramore Harris)

There was the anonymous man who made an ignorant remark about a child with Down syndrome; his father, Rob Scott, of Nova Scotia, posted a tearful video in response that went viral. There was the unknown truck driver who heckled British jogger Lindsey Swift about her weight; her biting open letter (“Engage your brain before opening your mouth”) was shared on Facebook thousands of times and landed her on the cover of a running magazine. There was the unidentified neighbor who reportedly told a transgender Girl Scout that “nobody wants to buy cookies from a boy in a dress.” After the scout’s foster mother wrote about the encounter, the story took off across social media and ended up raising thousands of dollars in cookie sales.

We may never know the name of Megan Harris’s mouthy saleswoman, but at least in this case, she was identified by her employer — an upscale department store chain concentrated in the heartland. It didn’t take long for Dillard’s to feel the sting. An executive from the chain called Harris to apologize and said the sales associate was “understandably mortified.” The Harrises, meanwhile, were swamped with thousands of encouraging messages from strangers and interviewed on TV; Lexi was even invited to be a model in a photo shoot for a prom dress vendor.

Other stories are vaguer, though — all those anonymous everyday villains — and some have been called into question.

Maybe you remember the waitress from a Red Lobster in Tennessee who posted on Facebook a receipt from a customer who seemingly left a racist message and no tip? It went viral, but then the customer stepped forward with a handwriting analyst to attest that it was not his penmanship — and promptly sued the seafood chain and the server.

Or you might recall the Long Island mother who supposedly declined her kid’s invitation to a birthday party hosted by two gay dads via a particularly nasty note: “I do not believe in what you do and will not subject my innocent son to your ‘lifestyle.’ ” Outraged social-media users shared a photo of the note far and wide — but it turned out there was no birthday party, no mean mom, just an entire story fabricated by two radio DJs, who were promptly suspended.

Even with non-hoaxes, there’s typically no way to discern the full set of facts — or get the other side of the story. Consider the nameless man who snarled the evening rush hour by pulling the emergency lever to exit a Metro train. He briefly held Public Enemy No. 1 status in D.C. Twitter circles — until fellow commuters revealed he had good reason to panic: Not because he had missed his stop but because his young child had been accidentally left behind on the platform.

Still, a clear lack of certainty about these stories doesn’t stop people from eagerly jumping on the bandwagon. Why?

Studies have shown that as social media bombards us with emotional stimuli, we are drawn in particular to messages or posts that convey anger — more so than those expressing sadness, fear, joy or disgust. A message like Harris’s appeals because people want to offer support — but also because of its personal resonance, researchers say.

“Maybe sharing it makes you feel better about the times you were hurt and didn’t do anything,” says Ryan Martin, an anger researcher and psychology professor at the University of Wisconsin at Green Bay.

Of the “thousands and thousands” of messages Harris received after she posted her letter, many were deeply personal: One woman shared that her own mother had called her fat and she wished someone had told her it was okay to be a size 12. Girls wrote to say that they used to cut themselves because they’d hated their teenage bodies. One mother wrote about her son, who committed suicide after a painful struggle with body dysmorphia.

“We were shocked,” Harris says. “This was a huge thing for so many people.”

It probably helped that Harris’s post featured a nameless perpetrator, Martin says — giving cover to Facebook followers who’d feel uncomfortable heaping criticism on an identifiable person.

And then there’s the opportunity to showcase your praiseworthy morality. “It’s a display to one’s Facebook friends to show your altruistic tendencies, what a kind person you are because you noted this,” says Charlotte Blease, a cognitive scientist at the University of Leeds.

Even in the digital realm, our behavior is still governed by the same primal instincts that guided our ancient forebears, Blease says. Members of ancestral tribes were hard-wired to focus on the most attractive, the strongest, the most powerful among them — because mimicking their behavior aided survival. In modern Internet culture, she says, this mostly means that we spend too much time retweeting Kim Kardashian.

But Blease argues that some viral morality tales have actual value.

“What’s interesting in [Harris’s] case is she’s using the power of the Internet to stigmatize the stigmatizer,” Blease says.

“Imagine if we lived in a really introspective society where people were constantly reflecting on their behavior?” Martin adds. “It could be a real teaching moment.”

That’s all Harris wanted from her post, she says. She never wanted the sales associate to lose her job or be punished.

“It was intended to empower my child,” she says, “and to make people, not just that particular lady, think about how we speak, especially to our children.”