The Progressive car insurance company has found itself on the wrong side of the Internet.

Matt Fisher, the brother of a former Progressive customer killed in a 2010 car accident, recently decided to publicize the company’s fight to avoid paying a $75,000 insurance claim to his family. Adding insult to injury, he wrote in a blog post Monday, “the guy who killed my sister was defended by Progressive’s legal team” during the trial. By Thursday, Fisher’s post sparked a national news story—and a debate over insurance practices—with coverage from the Associated Press, CNN, CNBC, a range of web outlets and an in-depth interview on CBS This Morning. The segment is well worth watching for the history of the case.

But how did this local story catch fire online and draw national attention?

The first key was the type of platform that Fisher used. He wrote his story on Tumblr, the popular blog and photo-sharing website. Unlike most blogs, which openly allow comments by any readers, Tumblr enables people to react to items only if they post a response by resharing the content on their own Tumblr page. It’s a subtle but significant shift in the site’s architecture, making it more of a dynamic network than a passive media platform. By raw numbers, that means Tumblr posts often draw fewer reactions than media websites. Yet when people feel compelled to respond or criticize an item, Tumblr fosters rapid and automatic broadcasts across people’s networks. A popular item at Huffington Post, for example, can draw more than 25,000 shares. Fisher’s Tumblr post, provocatively titled, “My Sister Paid Progressive Insurance to Defend Her Killer in Court,” had only about 11,000 shares by Thursday. But that was more than enough to get people in his network talking to their networks.

Fisher’s particular network was also critical. Plenty of worthwhile arguments and efforts are posted online, of course, and then lost in the web ether. Fisher is a Brooklyn comedian, and while he’s not famous, he already had a small network that follows him and, presumably, trusts him. So they took his story seriously and are probably shared it based on pure outrage and solidarity—without even calculating whether this was an “effective” strategy to change the company’s actions.

Fisher also kept his 2,000 Twitter followers updated as his efforts gained traction. So when Progressive responded in public, with a legalistic rebuttal arguing that it “did not serve as the attorney for the defendant,” Fisher tweeted out his Tumblr response. He also told his followers when he joined Glenn Beck’s radio program to talk about the (presumably bipartisan) outrage over the story. And he shared memories of his sister, Katie Fisher, from posting old pictures of her to recounting how she “slept over in my apartment the night before I got married and got me to stop pacing around.”

Reflecting on all the attention this week, Fisher even joked that he should have replaced his Twitter photo—a goofy headshot with his mouth agape—before becoming an insurance crusader. “Here’s a good idea for a week ago,” he tweeted, “how bout I change my twitter picture to something non-stupid-looking?” Of course, Fisher’s story and efforts moved people because he was a real person, dealing with both an extremely personal loss and a frustrating bureaucratic battle.

In the end, we know Fisher was effective because, after years of fighting, Progressive announced an agreement with the family on Thursday. That shift, however, came after a final bout of counterproductive corporate pushback in the very same week. Progressive had been repeatedly tweeting an automatic message to critics who contacted the company about the case.

“We fully investigated this claim and relevant background, and feel we properly handled the claim within our contractual obligations,” the company wrote, adding, “we’re sorry for everything Mr. Fisher and his family have gone through.” That cold reply, tweeted in a repeating loop, personified the company’s inhumane approach to many critics, drawing more outrage and online scorn. By Thursday, though, Progressive left the tweeting to others.

The company simply retweeted a news post from a CBS journalist, reporting that the Fishers “could receive settlement money” from Progressive “in the next days’ as soon as signed papers are exchanged.”