It appears that some people have their Users folders still visible after installing the 10.9.3 update. However, it also appears that when they are visible that the permissions are set for universal Read and Write which is not secure.
With this in mind even those that have their Users folders visible you should still boot into the recovery HD and change the permissions.
Here are the steps for both scenarios to fix this issue.
If the Users folder is visible. Start your computer while holding down the Command and R keys.
From the Utilities menu select Terminal.
In the Terminal type in the following commands:
cd /Volumes/Macintosh\ HD (or the name of your partition)
chmod 755 Users
chmod 755 Users/Shared
Quit the Terminal and restart your Mac.
====================================================
If the Users folder is not visible. Start your computer while holding down the Command and R keys.
From the Utilities menu select Terminal.
In the Terminal type in the following commands:
cd /Volumes/Macintosh\ HD (or the name of your partition)
chmod 755 Users
chmod 755 Users/Shared
chflags nohidden Users
chflags nohidden Users/Shared
Quit the Terminal and restart your Mac.