First, you can send the complaint by post (firstname.lastname@example.org), but then Meta may not acknowledge receipt. Stiftung Warentest recommends waiting three days for returns, after which all that remains is the printout and the good old registered letter with return receipt, which you should send to Meta’s European headquarters in Ireland.
The cost of this varies depending on the weight. In addition to the international postage for the letter (from 1.10 euros), there is 3.50 euros for international registered mail and 2.20 euros for the acknowledgment of receipt. With a standard letter, the whole thing costs €6.80.
If Meta can still manage, the 6.80 euros are gone. But you don’t have to act right away. According to Stiftung Warentest, the claim for damages is time-barred end of 2024 at the earliest.