It's for courts to decide when it gets challenged, but the general idea is, the less obvious or expected the agreement is for normal usage, the less likely it'll hold. So burying some text that amounts to "we get to hack your device" is about as likely to hold as "we get your first born child" (which has been done multiple times by different companies, for fun, and no it doesn't hold).
I come to believe, therefore if the subject of a contract, in the facebook case information theft and violation of privacy, the contract can be nullified in favour of the citizen
Unconscionability in german law may occur in a Knebelvertrag (oppressive contract) or in contracts that abuse power differences between the contractual parties or even usury (immoral monetary loans for example). In that case the contract can become invalid.
In case of any disagreement, the spirit of a contract weighs harder than the wording in textform, if it is possible to determine.
A contract can also not place any part of the agreement outside the reach of a court (ie via arbitration clauses. You can still fight your agreement outside court but you can't forbid someone from going to court). You can't waive most of your rights either and even if you do, a court may determine that contract to be invalid and the other party having violated a right will then be prosecuted.
Contracts themselves are largely a specific case of a legal transaction (negotium juridicum) and aren't specifically regulated (all above is basically just about any legal transactions ranging from contracts over last wills to verbal agreements).