There are more than 200 million active users currently accessing Facebook through their mobile devices.

"Mobile
There are more than 200 million active users currently accessing Facebook through their mobile devices.
People that use Facebook on their mobile devices are twice as active on Facebook than non-mobile users.
There are more than 200 mobile operators in 60 countries working to deploy and promote Facebook mobile products"

Source:  Official Facebook Statistics, retrieved 4th November 2010.
Earlier figures here (other figures on the Official stats, like total users, are unchanged)