Neither Linux nor Lemmy are a multi-billion dollar company.

If the advertisements were the product, then the exchange would be give ad, receive money. The advertisers are both giving the ad and the money to Meta. The thing the advertisers receive for giving the money are the potential customers. Meta is exchanging money for users. You are the product.

Meta’s entire model is categorizing the users so effectively and giving the advertisers the tools to target the users who are most likely to spend money once they see the ad. The advertisers pay Meta for access to users as well as the data about all of the different ways that groups of users are categorized. Then the advertiser can make a new ad or new product that will appeal to either a wider audience, an audience that is willing to pay a far larger amount of money than something costs to produce, or both.

The users and their data are the product of nearly every profitable business that provides something free to users. It’s up to you to decide how you feel about that. Maybe you see an ad for something and think “That’s exactly what I’ve been looking for!” and happily pay for it. That’s not necessarily a bad thing.