Reuters asked research firm Pathmatics to compile some estimates.
They conclude that spending on the site by the top 30 advertisers dropped 42% over November and December.
The estimate is based on technologies that track ads on desktop browsers and the Twitter app.
Pathmatics says it doesn't account for some factors, including incentives that advertisers might have got from Twitter.
That means the spending data could be higher for some brands.
Among big names...
Pathmatics found that Coca-Cola halted all spending on Twitter ads in mid-November.
Food giants Kraft Heinz and Nestle also stopped.
And retailer Target skipped advertising for Black Friday.
But Apple and PepsiCo actually stepped up spending.
Neither Twitter nor any of those firms would comment on the analysis.
But a drop in advertising would tally with separate reports by The Information.
It says Twitter revenue fell about 35% in the fourth quarter due to a slump in ad sales.
That may all make grim reading for Elon Musk.
Because coming up on his to-do list are the interest payments on the big debt incurred to buy Twitter.
