And if the user understand s/he's entering into a contract with Facebook.

It's very unusual to digitally/personally sign a contract when entering a relationship with a site like Facebook giving away a service for no money. It's very usual or inevitable (and expected) to do it when starting an online bank account or for a car sharing service, internet and phone providers, even a subscription to music/movie streaming or online news, something you pay for. There are usually a lot of forms to fill to enter into those contracts and that makes those contracts very visible and understood.

I hope this will be overruled at EU level.