Julianne Moore/Twitter

Spurred by grieving father Richard Martinez, "Not One More" became the new rallying cry against gun violence last week. And, it has since gone viral on social media.

Speaking to reporters shortly after his son, Christopher Martinez, was murdered in the massacre that left seven people dead in Santa Barbara, Calif., on May 24, Martinez fought back tears and blasted gun laws in the US.

"Why did Chris die? Chris died because of craven irresponsible politicians and the NRA. They talk about gun rights. What about Chris's right to live? When will this insanity stop?" Martinez said. "We don't have to live like this. Too many have died. We should say to ourselves, 'Not one more.'"

Later, at a memorial service last week, Martinez said he was going to "ask every person I can find to send a postcard to every politician they can think of with three words on it: Not One More."

Gun control advocates heeded Martinez's advice and ran with it. One group, Everytown for Gun Safety, created an online tool that lets users input their data and then the group sends the postcard for them. Everytown widely publicized this postcard campaign on Facebook and Twitter.

As of Monday, Everytown said it had received more than 500,000 sign-ups and it's now sending more than 2 million postcards to people's governors and members of Congress.

Politician, actors, and activists have also posted photos of themselves on Twitter, Facebook, and Instagram holding signs with the hashtag #NotOneMore. Within 24 hours of Martinez asking students to tweet #NotOneMore last week, the hashtag had been posted to the social network about 30,000 times, according to Mashable.

On Tuesday, actor Julianne Moore sent out an e-mail, via Everytown, asking people to take pictures of themselves holding signs that say "Not One More" and share them on Facebook and Twitter. If people opt-in, Everytown will also share the photos on its website using a #NotOneMore Flikr feed.

"Now you are helping to put [Martinez's] words into action," Moore wrote.

CNET contacted Twitter for up-to-date information on how viral #NotOneMore has been on the social network and we'll update the story when we hear back.