There are more than 250 million active users currently accessing Facebook through their mobile devices

"There are more than 250 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:  Facebook official statistics, retrieved 19th April 2011

Earlier figures here - 200m mobile users.  Other official data (e.g. number of users) has not been significantly updated.