There is a huge difference between selling your personal Facebook account and farming up likes and whatnot (e.g. accounts with a few thousand "friends") and selling pages. I was referring to the first one. Do you just go out there and sell your personal FB account when you need cash? You don't.
Perhaps Facebook isn't the best example, because FB has a real name policy and also if you sold your personal FB account then your friends would obviously know it was sold and the account would not be worth much, also if it was reported there is a high chance of the account being banned and you may not want your real name associated with the account after it was sold. But people sell their personal accounts on social media websites where they primarily interact with people they don't know in real life, Twitter is a prime example.