FB is an advertising firm, and the identifiability of their audience is a major value driver of their ad inventory[1]. They explicitly leverage the PII stored on their consumer users to enable advertising offerings such as Custom Audiences[2][3][4] and Advanced Matching[5][6].
Best practice for any B2C app like FB would be to retain those prior values indefinitely, since prior values still provide valid utility for their advertising services as additional data points for matching.
That likely differs from best practices for large B2C apps that aren't like Facebook and don't have similar incentives for indefinite data retention.
[1] https://www.adexchanger.com/mobile/facebook-could-take-a-mul...
[2] https://www.facebook.com/business/help/341425252616329
[3] https://www.facebook.com/business/help/2082575038703844
[4] https://www.facebook.com/business/help/2082575038703844