Mobile accounts for an estimated 20% of Facebook's ad revenues

"Based on an analysis of more than two million Facebook ad clicks and conversions, Kenshoo Social data show mobile now represents 20.3 percent of overall Facebook ad spend.
Other key findings presented by Kenshoo Social include:
Mobile ads on Facebook carry a 70 percent price premium over desktop ads
When comparing Apple iOS to Google Android on tablets, iOS had a 97 percent share of Facebook advertisers’ ad spend
When comparing Apple iOS to Google Android on smartphones, Android had a 71 percent share of Facebook advertisers’ ad spend"

Source:  Blog post from Kenshoo Social, 7th January 2013

Note - These are estimates