There will be an empty seat among first lady Michelle Obama’s guests at the State of the Union on Tuesday night in remembrance of the victims of gun violence, the White House announced last week.

Americans flocked to social media Monday, using the hashtag #EmptySeat, to remember loved ones they have lost to shootings. The initiative is being led by Everytown for Gun Safety, a New York-based nonprofit that advocates for increased gun-control measures.

Among the participants was Nelba Marquez-Greene, the mother of a 6-year-old girl who died in the mass shooting at Connecticut’s Sandy Hook Elementary School three years ago.

Marquez-Greene tweeted a photo of her late daughter standing on her husband’s lap. The girl wears a neon-pink shirt and matching headband, jean shorts and a calm smile.

In an interview with NPR shortly before the first anniversary of the of her daughter, Ana, in 2013, Marquez-Greene recalled that their last dinner together as a family had been a joyful one at the Cheesecake Factory, where they took pictures and helped themselves to two desserts.

“I’m so grateful we had that,” the mental health counselor said. Ana, her son, Isaiah, her husband, Jimmy, and her — all together at one table.

Now an outspoken advocate for violence prevention, Marquez-Greene tweeted Monday: “Tomorrow we leave an #emptyseat for Ana.”

There were remembrances from others, too. A father whose 20-year-old son was among those killed during the mass killing spree in Isla Vista, Calif., in 2014. A woman whose sister was shot and killed by an ex-boyfriend. Someone whose father was “murdered by a crazy person who bought a gun just by showing his license, no background check.”

Rita Everstine posted a photo of a Washington Post article from 1991, depicting her late older sister, dead at 14 when Everstine was 3-years-old.

In 1955, her sister, Michael Ann Ryan, was walking with another teenage girl going to pick up her report card on the last day of school. Their killer hid in a picnic grove near the University of Maryland and opened fire from 150 feet away.

Michael was fatally struck in the chest, ankle and back. “The killer was a fine shot,” The Post’s Phil McCombs wrote.

The last time a seat at the Seat of the Union was intentionally left empty was in 2003, when Laura Bush’s box had an open seat symbolizing “the empty place many Americans will always have at their tables and in their lives because of the attacks on Sept. 11, 2001.”

Obama’s announcement about the empty seat planned for his final State of the Union address came at the end of a week in which he issued an executive order to implement new restrictions on gun sales across the country.

The provisions, though modest, have been criticized for taking attention away from mental-health initiatives and the enforcement of current laws, as well as for infringing upon the Second Amendment in the eyes of some.

Some critics seized on the popularity of the #EmptySeat hashtag to express their displeasure with Obama and to comment on hot-button political issues other than gun control and gun violence.