If you are issued a document and then manipulate that document (especially if you do so with fraudulent intent and then pass it on as authentic), you are committing Urkundenfälschung ("forgery of documents") which can be punished with up to five years of prison or a monetary fine.
This applies to everything from government-issued identification to sick notes. The exact punishment likely depends on the intent and the type of document in question but forging government IDs is almost certainly a crime.
OTOH Facebook would have to obey certain laws if they want to check your ID, especially as they ask you to send you a full unredacted copy. Germany has a law book called the "Personalausweisgesetz" which defines who may ask for ID and how it has to be handled. That's aside from the privacy laws which already define rules for companies that handle or store personally identifiable information.
Arguably faking a government identification in any context is damaging to the credibility of that ID in any context. I still don't think it should be illegal, but the argument can be made rationally.
It's not uniquely illegal (at least in Germany). It's illegal for the same reason forging any document (for fraudulent purposes -- i.e. in 99% of all cases) is illegal: it misrepresents an authoritative claim.
It's not entirely unlike having a friend show up in a fake police uniform to vouch for you.
There are edge cases like art or educational purposes, but using a fake ID to prove your identity to a third party is definitely fraudulent: you're saying "this authority attests that this is my identity" when the authority in question does nothing of the sort and the document claiming it does is forged.
If you are issued a document and then manipulate that document (especially if you do so with fraudulent intent and then pass it on as authentic), you are committing Urkundenfälschung ("forgery of documents") which can be punished with up to five years of prison or a monetary fine.
This applies to everything from government-issued identification to sick notes. The exact punishment likely depends on the intent and the type of document in question but forging government IDs is almost certainly a crime.
OTOH Facebook would have to obey certain laws if they want to check your ID, especially as they ask you to send you a full unredacted copy. Germany has a law book called the "Personalausweisgesetz" which defines who may ask for ID and how it has to be handled. That's aside from the privacy laws which already define rules for companies that handle or store personally identifiable information.