I wouldn't be surprised if lawmakers set to make it so everyone had the right to make sure their information was not sold to advertisers.
I'm personally under the impression that regulation causes more problems than it solves in the majority of situations so I'm hoping that the industry self-regulates and companies like Facebook and Twitter allow me to pay for their services and opt-out of their business of selling my private information to others.
I'm not really sure why you defined "customer" for me. I never said that Facebook and Twitter users are currently customers, but that they should be.
It is dishonest to have the business taking place "on the back end" of things and to purposefully hide such transactions from people.
I also find it arrogant to think that it is advantageous to shield such financial complexity from people because they don't want to think about it or won't understand it.
So if Facebook and Twitter want to continue to be dishonest and arrogant companies, by all means, let them continue, but I think it will be part of their ultimate demise, either from increased consumer awareness on topics such as information privacy and digital identity, or from legislation.
I guarantee over 99% of Facebook's billion+ users have no idea that's really how Facebook is making their money. Sure, they might see the ads, but they don't know how those ads really work.
As for calling users "customers" because their data is being sold, well, no, that's not an accurate name for them per the definition of "customer":
cus·tom·er/ˈkəstəmər/
Noun:
A person or organization that buys goods or services from a store or other business.
A person or thing of a specified kind that one has to deal with: "a tough customer".
The label "customer" implies a conscious action of making a purchase. Twitter users are not actively purchasing anything, they're just creating usage data that makes money on the back end.