The Ubuntu Popularity Contest is a project from Canonical to measure statistics of usage of different apps and software in Ubuntu. It is disabled by default, but once enabled via software center preferences, a background program submits anonymous information about different package usage every week. The statistics are compiled, and based on it, decisions are taken which packages to include by default in next Ubuntu release.
Currently, over 3 million machines are submitting statistics via Ubuntu popcon. Based on it, we can get a rough idea about usages of different DEs in Ubuntu. The most popular DE of recent times is Unity with around 31% of market share. Gnome classic users form around 28%, KDE 22% and Gnome shell 11%. The high rate of Unity usage is can be though mainly because of its shipment by default in Ubuntu. Of the total number of users, classic Gnome occupies 59%, Unity 15%, KDE 9% and Gnome shell 4%.
77% users use 32 bit architectures, and 21% use 64 bit
The Linux subreddit has a nice discussion going on about the stats.