The march has precedent in the annals of online activism: the Arab Spring demonstrations of 2011, and the Tea Party, Occupy and Black Lives Matter movements, for example, were all driven by social media.
But perhaps no such effort has turned out so many in a single day. The crush of bodies was so heavy that organisers and public-safety officials in several cities, including Chicago, suspended plans for actually marching anywhere. That turned some of the gatherings into rallies.
As with those other grass-roots causes, traditional news media outlets were late in catching up to the story.
"NBC Nightly News" did its first story about the march on Friday, two days before hundreds of thousands took to the streets, according to a search of the Nexis database.
ABC's "World News Tonight" aired an 18-word sound bite from one of the march's co-organisers on Thursday. And though the New York Times mentioned the event numerous times in the weeks preceding the march, the paper featured it just once on its front page last Wednesday. (Its story concerned racial divisions among organisers and marchers).
"The women's marches were pretty much under the radar in most mainstream media coverage over the last few weeks," says Marcus Messner, an associate professor of journalism at Virginia Commonwealth University, where he studies social media. The number of demonstrators and events, he says, "caught the media and public off guard," even as the social-media buzz began growing into a "huge groundswell".
According to Messner, the event demonstrated that "organisers don't need media coverage anymore to reach large audiences and turn out large crowds for protests when people are passionate about issues and connect via social media".
TV reporters spent much of yesterday marvelling at the massive crowds gathered in Washington, Boston, Chicago, Los Angeles, London and other cities. "The Metro system is completely overwhelmed. The cellphone system is overwhelmed. The satellite trucks are overwhelmed," reported MSNBC correspondent Cal Perry from the Mall in Washington, adding that, "We're looking at a city that's overwhelmed."
A few minutes later on CNN, reporter Jessica Schneider also invoked the o-word: "The turnout here in New York City, frankly, is overwhelming," she said, as thousands could be seen behind her, shuffling down a city street.
Cable news coverage toggled between the march and Trump's activities during his first full day as president, including attendance at a morning prayer service and a visit to CIA headquarters in the afternoon. Fox seemed to go heavier on the latter: hosts Bill Hemmer and Shannon Bream took a few minutes in the afternoon to speculate about Trump's potential choice of White House pet and to show Donald Trump jnr's video of his family bowling in the executive mansion.
The march itself began as a single Facebook post the day after Trump was elected on November 8.
Disappointed by Trump's victory, Teresa Shook, a retired lawyer who lives in Hawaii, dashed off a post asking if women were interested in rallying in Washington around Inauguration Day.
She asked her online friends for help creating an event page, and posted one for her proposed march. When Shook went to bed that night, 40 people had signed on to the idea. When she woke up the next morning, the number of responses had jumped to 10,000.
New York fashion designer Bob Bland had a similar idea around the same time. She proposed a "Million Pussy March," naming her creation after the vulgar language Trump used in describing his treatment of women during an unguarded 2005 conversation with "Access Hollywood" host Billy Bush.
By early January, more than 150,000 people had responded to what professional organisers had renamed the Women's March on Washington. Dozens of groups, including Planned Parenthood, soon became partners, spreading the news on Facebook and other social-media platforms.
"Social media has entirely changed the organising landscape," says Karen North, the director of the social-media programme at the University of Southern California's Annenberg School for Communication and Journalism.
"It is a way to ask people to join with friends and with like-minded people and promote a sense of belonging. Social media allows us to organise people in a manner that feels like a personal invitation, and also in a manner that suggests a groundswell of support and passion about a cause."
Even if they don't spark movements, Messner says traditional media outlets can still act as "an amplifier" of them, spreading attention and in some ways validating them. "It's not unlikely that women who stayed home today will show up next time, as they saw that they will be part of a bigger movement." Mainstream media coverage, he says, "can now lead to an even bigger turnout down the road, if the movement continues".
If it does, the news media will have learned a lesson from the weekend event, he suggests. Before the next big march, according to Messner, mainstream media coverage will be "guaranteed".
