$Twitter (TWTR.US)$ is struggling to keep its most revenue-generating users engaged, highlighting a problem for $Tesla (TSLA.US)$ CEO Elon Musk as he nears the completion of his $44 billion acquisition.

What Happened: The top 10% of users — who account for 90% of Twitter's earnings — are known as "heavy tweeters," and they have been in "absolute decline" since the COVID-19 pandemic started approximately 2 1/2 years ago, according to a Reuters report that claimed to have seen key internal Twitter research.

According to the Reuters report, the most active English-speaking Twitter users have changed their interests over the previous two years, which may make the platform less appealing to advertisers. The shift in interest has seen growth in the number of heavy users interested in NSFW (not safe for work) and cryptocurrency content.

The English demographic is reportedly important for Twitter, as the social platform generated more ad revenue from the U.S. alone than from all other markets put together in its most recent quarter. But, "Twitter declined to specify how many of its tweets are in English or how much money it makes from English speakers," the report said.

Why It Matters: In a video chat with debt bankers who are lending money to the deal, Musk reportedly "pledged" on Monday to "close" the deal for Twitter by Friday, according to a prior article from Benzinga.

The Reuters report came days before Musk intended to close the deal; if the information in the report was true, and not yet seen by Musk, it would fuel worries that the platform would be even less profitable given Twitter's ad revenues account for a large portion of its total bookings.