Comment 14 for bug 881098

>I had another type of error on 12.04, the 'users' command used for checking of logged users doesn't return all active users in the Unity.

Not only Unity, any X session not shown by "users" command. Xfce, gnome-fallback - does not matter.

Actually, it is problem not with "users" command itself, but with lightdm, that doesn't update /var/run/utmp.

  ck-history --last-compat|sed -n "s/\([^:]*:[^ ]*\).*still logged in[ ]*/\1/p" | cut -d' ' -f 1 | tr '\n' ' '

Command above works for X sessions.