Ads, but without targeting. They sell the search terms entered in an anonymized way. So you can buy an ad that will display to everyone who searches DuckDuckGo for "vacuum cleaner". You cannot buy an ad that will display to women aged 30-40 who live alone in a zip code where the median income is above $100k when they search "vacuum cleaner".
What's interesting, does that strategy end like: less effective ads -> need more of them -> raise ad price + show more of them?
Another thing is regarding k-anonymity which I suppose privacy-first guys would use. If I gather user info with a high entropy, I don't have an incentive to sell it to anybody and create a competitor, and it'd probably illegal to do that. But if I anonymize the data set into a k-anon data it's now less restrictive to sell from legal point, also I'll probably sell that data to third parties just to make ends meet.
> Weinberg: We make money the same way other search engines make money. When you type in “car,” you get a car ad. We syndicate Microsoft and Yahoo ads through the Microsoft Yahoo Search Alliance.