> Something a little more high tech, like running MySQL on top of a file system that does synchronous replication.

You can't guarantee a consistent snapshot from the file system alone for Innodb. You need some other logic on top which is exactly what Xtrabackup provides.

RDS certainly isn't going to work for Facebook, but in order to achieve their fast snapshot feature RDS is almost certainly using LVM snapshots.

LVM snapshots can be a better option than mysqldump but LVM snapshots are local and therefore require enough space to take the snapshot and keep it for the time it would take to copy it off box. This can be a big problem as you can't rely on your snapshot to always work. Also, using an LVM snapshot requires a 'flush table with with read locks' command to be run while the snapshot is in progress which can bring in its own issues.