An online account is a different thing from a physical object like a letter or journal, which can be freely transferred from one living person to another living person. (The right to reproduce the contents of letters and journals is another matter!) An online account is a contractual relationship. I don't know about Facebook, but most online accounts are explicitly non-transferable. So it's not at all obvious that Germany should make these things transferable on the contractee's death regardless of what's written in the T&Cs, if that's what's happened.
Contractual relationships have limited conditions under which they can be terminated. If you die, your heirs (or in this case parents) can inherit the contract and continue it.
Additionally, not everything the T&C says, up to and including "accounts are not transferable" is not automatically legal.
Sometimes they are inherited, sometimes not, depending on the T&Cs. If it's not transferable while the contractee is alive then I wouldn't expect it to be automatically transfered when the contractee dies.
