They know where you live, they know where you work. They know when you are home and when you aren't. They know every website you visit and for how long. They know what restaurants you eat at. They know who your contacts are, how often you talk to them and what you talk to them about. They can likely tell your sleep habits. If you do banking online, well you just gave them access to all your banking info too. Have you purchased anything online? They likely have your credit card info. Take pictures of your family and friends? They have every single one you have sent to anyone regardless of whether you did it on facebook, text, email, twitter, or any other way along with all the metadata that goes with it.
Does all that matter to you? Maybe not. But if a dishonest employee wanted they could do whatever they wanted with that info. Worse, Facebook almost certainly sells ALL that data to third parties. Someone could essentially steal your identity and bankrupt you. I do not trust Facebook employees with that data for a second let alone whatever shady third parties they sell it to. Can you really say all that risk is worth $20/month? I simply do not get it.