Rogues went from 7.67% of max level at patch 5.0.4, the pre-Mists of Pandaria patch, to 5.51% of max level as of patch 5.1, a drop of over 2%. This is at a time when most other classes either held steady (Paladins, Druids, DK's and Hunters all held at about even with their Cataclysm and patch 5.0.4 numbers), went up (Warriors saw a jump from 9.25% at max level to 10.14% between 5.0.4 and 5.1, while Warlocks went up from 6.7% to 7%) or saw slight declines (Shamans, Priests and Mages all saw slight declines). By comparison, the rogue decline becomes stark.
So, where have all the rogues gone? Monks have taken a slim 4.9% of the total playerbase, which means that they're hardly the dominant juggernauts that Death Knights became in Wrath of the Lich King, so can they explain the rogue decline?