Number one thumb rule, if you use an app without payment.
You are the product of it. This isn't just for faceapp but also for each app, they save our data and monetize with it.
100% I don't think the general public understands the workings of companies and how they would make money. If you don't pay, you are the product.
Mostly users doesn't know this is the real thing in the world of digital.
But I'm happy to see that there are people that are starting to be curious on how these apps earn and searches about the truth about it. Regardless of what they know, just spread the knowledge about these kind of apps and warn people whom you would deal with about these apps.