(REUTERS/Thomas White/File Photo)

Several of the most popular Twitter accounts, including Barack Obama and Katy Perry, lost millions of followers starting Thursday as the company began culling suspicious accounts, the latest effort to clean up the social media platform.

Twitter said on Wednesday it would begin deleting locked accounts this week, which it predicted would result in a “significant drop” in the number of followers for several of its biggest accounts. Twitter said the purge would take a week.

Obama lost more than 2 million followers by Friday morning, a little more than 2 percent of the 104 million followers he had at the start of the week. He is the third-most followed person on the platform, according to analytics company Twitter Counter. President Trump’s personal account shed more than 200,000 of its 53 million followers.

Singers Katy Perry and Justin Bieber each lost nearly three million followers. They are the two most popular Twitter accounts, with 110 million and 107 million followers respectively before the purge.

Television stars Oprah Winfrey, Ellen DeGeneres and Kim Kardashian West each lost more than a million followers. Global soccer superstar Cristiano Ronaldo lost 900,000 followers since July 4, wiping out the bump of a half million followers he gained during the World Cup.

Most of these accounts, however, saw their follower counts start to slowly tick upward by Friday morning, on track with a rate of follower growth that is not unusual for the respective accounts. Obama, for example, gained nearly 2,000 followers in the overnight hours, which roughly fits the range of followers he gained in previous weeks during the same Thursday-to-Friday overnight interval. Trump, on the otherhand, gained more than 14,000 followers overnight following the deletion of accounts, nearly double the rate he gained followers in a similar time interval in previous weeks.

Twitter, which has struggled to combat the prevalence of trolls, spam bots and disinformation, had been suspending accounts at the rate of over one million per day in recent months. The increased rate of suspensions throws into question Twitter’s estimate that spam or troll accounts make up only a small fraction of its users.

Vijaya Gadde, Twitter’s Legal, Policy, Trust and Safety Lead, wrote in a blog post on Wednesday that “the most significant changes are happening in the next few days, (and) follower counts may continue to change more regularly as part of our ongoing work to proactively identify and challenge problematic accounts.”

The company said the locked accounts slated for deletion are not necessarily the same as spam accounts or bots, but are most often accounts that were created by real people which Twitter locked following suspicious changes in behavior. The locked accounts were dormant for at least a month awaiting a password change by the owner, so it should not affect daily or monthly user metrics, the company wrote.

Twitter’s advertising-centric business model depends on metrics that show user activity, and the deletion of so many accounts can negatively affect those numbers. Twitter’s stock plunged nearly 10 percent after The Washington Post reported Twitter had been locking accounts, and has been slowly rebounding since.