Threads has been massively successful when it comes to signing up new users in its short history, but whether it’s retaining those users is up for debate.

A study from data-tracking site SimilarWeb finds that the number of daily active users has dropped from 49 million to 23.6 million in a week, meaning people who rushed to sign up aren’t coming back regularly.

That’s not worrying Meta CEO Mark Zuckerberg, though, who took to Threads Monday to express his optimism about the platform.

“Early growth was off the charts, but more importantly 10s of millions of people now come back daily,” he wrote. “That's way ahead of what we expected. The focus for the rest of the year is improving the basics and retention. It'll take time to stabilize, but once we nail that then we'll focus on growing the community. We've run this playbook many times (FB, IG, Stories, Reels, etc) and I'm confident Threads is on a good path too.”

The SimilarWeb data was taken based on Android usage, which the organization says is easier to track than iOS. (Apple numbers will be released in the coming weeks, it says.)

With 23.6 million active users, Threads’ audience is about 22% the size of Twitter’s. Daily usage on the app averaged 21 minutes on July 7. By July 14, however, that was down to about six minutes.

“Threads is missing many basic features and still needs to offer a compelling reason to switch from Twitter or start a new social media habit with Threads,” the group wrote.

Twitter user retention has also been declining, SimilarWeb notes, with the number of new users who continue to use it regularly falling from 19% in May 2022 to 16% in May of this year.

This story was originally featured on Fortune.com

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