One side effect of the iTunes 11.2 update on Thursday, May 15th 2014 has been that some but not all Macs were seeing the /Users and /Users/Shared folders disappear.
The permissions on the /Users folder were also changed to be world-writable, so that anyone could read and write to the /Users folder.
As far as bugs go, this is a very fascinating one. Initially, people thought the OS X 10.9.3 update was the culprit, but as it turns out - it's the iTunes 11.2 update. I'm interested to (eventually) hear the root cause of all this, but for now, the linked article contains a temporary workaround.