Interesting. I assumed FB would have hot replicas (kept up to date with live master-slave replication at all times) - ready to go any time a main database fails. Cascade that to another layer of slaves and there's no need to restore anything ever.
Replicas are for failover and read scalability. In terms of failover, when a master dies unexpectedly Facebook's automation fails over to promote a replica to be the new master in under 30 seconds and with no loss of committed data.
Backups are for when something goes horribly wrong -- i.e. due to human error -- and you need to restore the state of something (a row, table, entire db, ...) to a previous point in time. Or perhaps effectively skip one specific transaction, or set of transactions. Replicas don't help with this; as you mentioned, they're kept up-to-date with the master. So a bad statatement run on the master will also affect the replicas.
Occasionally you have some massive failure involving both concepts, like you have 4 replicas and they're all broken or corrupted in some way, then backups are helpful in that case as well.
In failure modes like accidental delete without WHERE clause, or a write that corrupts the validity of the business logic, it's useless, as you can watch the logs to see all your slave machines keenly and unquestioningly repeating the issue.
The wording is a bit cryptic, but it does seem that they definitely have hot standby-esque capabilities in place, in addition to long-term storage of their incremental/full backups.
[1] - https://www.facebook.com/notes/facebook-engineering/under-th...