"Unarmed 17-year-old boy shot by neighborhood watch captain in Sanford, FL outside of Orlando," the tweet said. It provided a web link to a story.
Since then, hundreds of thousands of messages have spread the word. Racial tensions have rapidly escalated as civil rights groups have held rallies, saying the shooting was unjustified.
Trayvon's parents started an online petition on Change.org demanding Mr Zimmerman's arrest. It has had more than 1.2 million signatures.
Filmmakers Michael Moore and Spike Lee have also posted messages in support of Trayvon.
The online uproar grew after the first emergency call tapes from the day of the shooting were released last week. In one of the calls, a dispatcher told Mr Zimmerman to stop following Trayvon, but he continued.
On Twitter, Trayvon has been mentioned almost 600,000 times, according to the social media monitoring firm PeopleBrowsr.
On Facebook, some protesters are wearing hoodies in their profile photos with the caption, "Do I look suspicious?"
Mr Zimmerman claims self-defence, saying he shot Trayvon after being attacked by him.
Martin, who lived in Miami, was in Sanford visiting family when he went to a corner shop for a snack. He was walking back carrying a bag of candy and can of iced tea, the hood of his jacket pulled over his head because it was raining.
He was approached by Mr Zimmerman, who told a police dispatcher he thought Trayvon looked suspicious. Mr Zimmerman shot Trayvon following a chase and fight.
The Justice Department and FBI have opened a civil rights investigation, and the local prosecutor has convened a grand jury April 10 to determine whether to charge Zimmerman.
"What you're seeing is that the Trayvon Martin case speaks to people around the country just like it speaks to people in this community," said Benjamin Jealous, president and CEO of the National Association for the Advancement of Colored People.
"It would have been easy for people here to say, 'He wasn't one of us. I didn't know him. My kids didn't go to school with him.' But instead, people here are saying what people said around the world, which is, 'He reminds me of my cousin, of my son, or my grandson.'"
Jeanette Castillo, an assistant professor of digital media at Florida State University, is tracking the Martin case on Twitter.
She said the case has played out in a protest era that will be increasingly driven by online audiences.
"You can hear about an issue in traditional media and be outraged. But in social media you have immediate feedback of how much your friends are outraged," Ms Castillo said.
"It's just a huge facet of social media that affects that mobilisation. It's sort of the same thing as word of mouth, but just at a lightning speed."
Recent research by the Pew Research Center's Internet & American Life Project shows why this case might particularly resonate for the black Internet audience.
Aaron Smith, a senior researcher for Pew Internet, said a study updated last month shows that 15 per cent of all Internet users nationally use Twitter, including 8 per cent on a typical day.
White users are generally in line with the national average with 12 per cent using the service or 7 per cent on a typical day.
By contrast, black Internet users have very high rates of Twitter usage, with more than a quarter using Twitter overall and 13 per cent using Twitter on a typical day.
"It's a bit different data than we've seen historically," Mr Smith said.
"For a long time, it was always digital divide story. But with social (media) we're finding the black community on par with or ahead of their white counterparts with usage."