In the summer of 2005, Alexis Ohanian, a tech entrepreneur, sent an email to his colleague Steve Huffman with an ominous subject line: “Meet the enemy.”

The body of the email contained just one line — a link to Digg, a community-focused social message board where people shared and discussed news articles and links to other sites they found interesting. Mr. Ohanian and Mr. Huffman, who had founded a similar effort called Reddit, set their competitive sights on Digg and its founder, Kevin Rose.

In the 20 years since, these entrepreneurs have gone onto other projects and, in true Silicon Valley fashion, dipped into other parts of tech. Along the way, Digg, which went from popular to not, all but died.

On Wednesday, Mr. Rose announced that he had bought back Digg for an undisclosed sum from Money Group, a digital media company, and would rebuild it to take on Reddit. And he is doing it with an unlikely ally: Mr. Ohanian.

“This is the perfect time to revisit this idea with fresh eyes,” Mr. Rose, 48, now a venture capitalist at True Ventures, said in an interview. He said social media had become so ubiquitous that “it doesn’t need to be winner take all,” adding that “we don’t need to take down Reddit to win.”

Mr. Rose and Mr. Ohanian, 41, are relaunching Digg when social media is in tumult. Elon Musk, who bought Twitter in 2022 and renamed it X, has turned the platform into a mirror of himself. Meta, which owns Facebook and Instagram, is becoming more video-focused to compete with TikTok. And Reddit, which went public a year ago, has added gamelike features to nudge users into spending more time on the site — and more time looking at advertising.