Compiz has updated to 0.9.x in Ubuntu’s universe repository, and the snow plugin has outdated. Now it’s difficult to let it snow on Ubuntu using Compiz.

Fortunately, there’s a small X window application called Xsnow that brings Christmas to your desktop.

Xsnow can be installed via Ubunu Software Center, or:

sudo apt-get install xsnow

Once installed, run xsnow in a terminal.

xsnow

Default number of snowflakes is 100 (max is 1000), it can be changed by -snowflakes. Default delay after updating everything is 50 ms, it can be changed by -delay.
Example, uses the maximum number of snowflakes and runs as fast as possible.

xsnow -snowflakes 1000 -delay 0

More usage, read:

man xsnow

BTW: you can make top-panel auto-hide by extensions.gnome.org (open via Firefox in gnome 3.2+), snowflakes may drop and build up on panel.