I was on the DBA team there around that MAU. I had lots of company and support from SRO (aka jr DBA's) as well as other teams (provisioning, etc...).

> Facebook with 400m users had just one DBAs if I remember correctly.

You don’t remember correctly. There are multiple and to my knowledge there hasn’t been just one in at least the past ten years.

Millions of Facebook accounts are a pretty harmless size metric. DBA workload increases with the number of employees, which is proportional to support requests, accidents, changes, strange needs that need to be addressed, and so on.

Buying and provisioning disks before space runs out is a small part of a DBA's job, and a modest constant-size task compared to predicting how fast space is running out.

Have you got a citation for that claim because that figure seems a little hard to believe.

In terms of your general point: it really depends on your business. In my last job there were 4 DBAs out of 12 total IT staff and they were constantly busy. In my current job there are no DBAs in a much larger team and yet no requirement to need one either. The two businesses produce vastly different products.