Take Facebook, for example: at least for my generation, when you don't have an account, you can sometimes feel like you are a burden for others, who organize everything within it (I don't have one and am doing fine, though).

Or take GMail: the only ways to avoid your private email being automatically processed by Google is either to avoid every person using GMail (well...), or to teach those person to use end-to-end encryption. Not that easy.