About 70% of Twitter’s top 100 ad spenders from before Mr. Musk’s takeover weren’t spending on the platform as of the week ending Dec. 18, according to an analysis of data from research firm Pathmatics. It’s a problem the billionaire can’t afford to ignore, since roughly 89% of Twitter’s $5.1 billion in revenue last year came from ads.

The advertiser exodus from Elon Musk’s Twitter Inc. is accelerating, and it could be hard for the billionaire to bring them back soon.

The lengthening list of advertisers pausing their spending on the social-media platform two weeks after Mr. Musk’s $44 billion takeover is adding to the financial pressure on the company. Mr. Musk spent the past week trying to assuage advertisers’ concerns, but many of them are defecting anyway.

On Tuesday, a coalition of more than 40 civil-society groups published an open letter to the 20 brands that spend the most on Twitter. The letter called on them to suspend their ad spending “if Mr. Musk follows through on his plans to undermine brand safety and community standards including gutting content moderation.”