Thursday's iTunes 11.2 release contained a bug that was both inconvenient and risky: If you have Find My Mac enabled, your Users folder (/Users) will be hidden at startup. Worse, that folder, along with /Users/Shared, will have their permissions set so that anyone can modify them, creating a security risk.
Our Christopher Breen explained a couple ways to make /Users visible, but that was just a workaround--and your changes were reset when you next restarted. The real fix came from Apple late Friday night in the form of iTunes 11.2.1. Install this minor update, and both the visibility and permissions of the /Users and /Users/Shared folder will be corrected--and will stay correct.
You can install iTunes 11.2.1 using the Mac App Store app, or by downloading the latest version of iTunes directly.