The saying that if you are not paying for the product you are the product is repeated so often that it has stopped conveying anything, which is a shame, because the mechanism underneath it is precise and worth understanding rather than sloganizing. A service that costs nothing still has costs, servers, engineers, bandwidth, and those costs are met somehow. When the user pays no money, the revenue comes from the data the user generates and the attention the user provides, and the service is engineered, deliberately and skillfully, to maximize both.
The revenue model has a specific shape. The service collects data about what you do, not only within it but often across the wider web through embedded trackers, and it uses that data to build a profile detailed enough to predict what you will respond to. That profile is then the basis for selling access to your attention, since advertisers in most cases do not buy your data outright, they buy the ability to reach the profile you fit, and the platform keeps the data because the data is what makes the targeting valuable. The more time you spend and the more you reveal, the more precise the profile and the higher the price your attention commands, which is why these services are designed to hold your attention as long as possible.
This is the part the slogan misses, that the business model shapes the product itself. Because revenue rises with engagement, the service is optimized to be engaging, and engaging is not the same as good for you, it is whatever keeps you scrolling, returning, and reacting. Features that would reduce time spent, however much they might benefit the user, work against the model and tend not to survive. The result is a product whose incentives are quietly opposed to your own, it profits when you use it more, whether or not using it more serves you, and the design decisions follow that incentive with considerable sophistication. This is why so many of these services feel engineered to be hard to put down, because they are.
None of this requires assuming bad intent, it is simply the logic of the model working as designed, and recognizing it is what lets you use these services on clearer terms. The choice is not necessarily to abandon them, since some are genuinely useful, it is to see the exchange for what it is and decide knowingly, to understand that the free service is paid for with your data and your attention, and that its design is optimized for the payer's benefit, which in this arrangement is not you. The people who use these tools well are the ones who never forget they are the ones being sold, and who spend their attention deliberately rather than having it extracted.