Because it works perfectly fine? I tend to write anyone off who scoffs at simple database-as-queue designs without understanding what the scaling requirements are. You can use a database as a job queue for 10s of thousands of jobs per day without any sweat.
Please forgive me, but I don't understand your hostility.
I understand that there are perfectly legitimate reasons for using a database as a queue. If you frequently need to look at and rearrange your jobs while they're in flight, chances are a pure FIFO structure probably doesn't work that well for you anyways. If the enqueue is contingent on a transaction committing, probably makes a lot of sense for the job to be in the same database. You don't need to tell me -- I've seen more than a few in production.
But an actual message queue also "works perfectly fine" based on the information provided, and I would imagine that a company like Facebook probably already has a few of those lying around. It would have been a conscious choice to use MySQL as a queue.
