It makes sense that when Facebook, LinkedIn et. al were set up social networks were something new so people wouldn't understand the benefits enough to pay.
Right now, we've seen what social looks like when you don't pay for it. You become the product.
I would have happily paid for WhatsApp instead of letting them give all my numbers to fb.
But the networks would still do it, even if they didn't have to. Look at Microsoft - you paid hard cash for you copy of Windows, and they still vacuum all your data and behave as if they were scummy cloud company.
Even if you pay, there's no reason for companies not to do this. Neither users nor regulators don't punish companies that spy on their customers and sell their data, so for the companies, it's free money. Why wouldn't take free money?
Sure, they could not be evil, but not being evil doesn't take you far in the business world - see every big company ever.