The “Barbie” star Margot Robbie created an account. Ditto Rian Johnson, the “Knives Out” auteur. Christopher McQuarrie, Tom Cruise’s directing partner, has used his to heap praise on another action star (Sylvester Stallone).

Letterboxd, the social network for recommending and reviewing movies, has become a kind of shibboleth for film nerds over the past decade. Roughly 10 million people now use the service to share their favorites: You like Studio Ghibli, too? What’s your favorite Spike Lee joint?

The service has not undergone any revolutionary changes since it was founded in 2011. But Letterboxd is undergoing two big changes: a new owner and, eventually, user recommendations and review of TV shows.

Matthew Buchanan and Karl von Randow, Letterboxd’s founders, announced on Friday that they were selling a majority stake in the service to Tiny, a public company in Victoria, British Columbia. The deal values Letterboxd at more than $50 million, said a person familiar with the sale, who spoke on the condition of anonymity to discuss confidential financial information.

Mr. Buchanan and Mr. von Randow, two entrepreneurs based in New Zealand, have reassurances for their users who may be afraid of what a sale could mean for their corner of the internet. First, neither co-founder is planning to leave any time soon, and both will remain shareholders. And the service itself isn’t changing immediately. The proposal to incorporate TV is still in its infancy, and the founders said they did not expect that the addition would disrupt their existing products.

“They’re just these incremental changes that make life better and easier,” Mr. Buchanan said.

Letterboxd grew quietly for many years, a relative oasis amid the quagmire of social media. Then the app’s popularity surged during the Covid-19 pandemic. By the end of 2021, Letterboxd had more than 4.1 million registered users, up from about 1.8 million in March 2020. Today, Letterboxd has more than 10 million registered accounts, about half of them active in the past 90 days.

It wasn’t long before that runaway growth attracted a suitor: Andrew Wilkinson, a founder of Tiny, reached out to Mr. Buchanan by email in March and made an offer for the company in May.

Though Mr. Buchanan and Mr. von Randow were reluctant sellers — they have raised no money and were not seeking a buyer — they said Tiny was a good match. Both are run by soft-spoken designers turned entrepreneurs who met on Tumblr and who speak reverently of the community of film nerds on Letterboxd.

Tiny, a holding company modeled after Warren E. Buffett’s Berkshire Hathaway, was born from a frustrating experience that Mr. Wilkinson had when selling his own web design firm in 2014. A former barista, he said he had been inspired to try his hand at building websites after some of his laptop-bearing customers told him that they made $500 for each assignment. After Mr. Wilkinson sold his business, and his acquirer ruined it, he got an idea.

“I vowed that if I ever had enough money, I would become the acquirer that I wish I could have sold to,” Mr. Wilkinson said.

Among the things that Mr. Wilkinson has no plans to change is Letterboxd’s business model. The service, which is free, has paid tiers for $19 and $49 a year for members who want extra services, such as the ability to pick their favorite posters for each film and change their user names. It also sells advertising and has a small team of journalists who produce a podcast and cover entertainment industry events.

As Letterboxd’s business has expanded, its staff has grown to match. There were no full-time employees when the pandemic began, Mr. Buchanan said. Today, the site has 16 full-time employees and a dozen part-timers, making it an opportune time for a new owner to provide support.

“We’ve done a really good job at giving the outward appearance of a larger organization,” Mr. Buchanan said, adding, “I think we’re just looking for the next stage of support.”