It is a marketing strategy that in starting companies give access to their platforms free of cost and after sometime when they get users they start charging them as we see Facebook charges amount when users verified their profiles same goes with snapchat. And yes Facebook sell advertisements space to advertisers and in return Facebook generate revenue.
That is right, it is a marketing strategy and I think everyone is shown to the terms and conditions before they sign up. The revenue that they generate is because of the users but they have infrastructures that are open to everyone and I think that's the best product that they can offer for free in return of their revenue. Whilst for the verification, it's not that much at all because they have copied the idea to Twitter now X to have those verified badges but it's another source of revenue for them.