Ask HN: For Facebook, what's the most efficient way to erase data from backups?

Mark Zuckerberg just said to the Congress that Facebook erases users' data from their backups when a user deletes his account.

For 1 small database, I'd restore the dump, delete the data and redump without the data. But at the scale of Facebook, what's the most efficient way to achieve it?

I don't know in how many instances of MySQL my data are, or what's the backup periodicity & retention time. But let's say my data are in 1 MySQL, backed up every day, kept for 1 week. That makes 7 dumps of several petabytes. Let's say I have also some data in other DBs (Cassandra, Redis, etc). And let's say there are 10,000 users who delete their account every day. How Facebook does it?