In Wigan, a longtime Labour stronghold in northwestern England where the Conservatives have been trying to capitalize on strong pro-Brexit sentiments, newspapers and television remain a common way for many people to get information. But for others, like Greg Rimmer, a 26-year-old who works for the local government, everything comes from social media.

Pulling out his phone, he scrolled through a Twitter feed filled with political posts criticizing the Conservative Party’s health care plans. He tapped over to Facebook, where he saw another meme about the election.

“I hardly ever watch the news, but I’m on Facebook, Twitter, Instagram and YouTube all the time,” he said. “I try not to let it sway me, but if you see something constantly, you can’t help it.”

Many videos shared on Facebook and in WhatsApp groups around the country originate in a cramped office in the northern London neighborhood of Finsbury Park.

Emil Charlaff, who helps lead that effort at Momentum, a pro-Labour group, said social media played a critical role in building support and organizing voters.

The group has about 15 people who produce videos, memes and other social media content, often using humor or outrage to grab attention. “People are so saturated, you need to grab their attention in the first three seconds,” said Mr. Charlaff, who said Momentum did not knowingly publish any false information.

Britain’s election is the first major campaign since Facebook said it would not fact-check political ads from candidates and political parties. Sam Jeffers, a co-founder of Who Targets Me, a group that tracks Facebook political advertising, said that since then, there had not been the flood of disinformation that many had feared.