https://www.cnet.com/news/facebook-proof-cambridge-analytica-deleted-that-data-was-a-signature/

For Facebook, a signature from Cambridge Analytica whistleblower Christopher Wylie was enough proof -- a decision that would come back to bite the social network.

When Facebook learned about Cambridge Analytica buying 87 million people's profile information without user permission in 2015, the social network's executives said it requested the data be deleted. Facebook said that it received "certifications" that had happened, but now says it's learned the UK-based data profiling firm lied.