Mobile accounts for 78% of Facebook's ad revenue

"Third Quarter 2015 Operational Highlights
Daily active users (DAUs) - DAUs were 1.01 billion on average for September 2015, an increase of 17% year-over-year.
Mobile DAUs - Mobile DAUs were 894 million on average for September 2015, an increase of 27% year-over-year.
Monthly active users (MAUs) - MAUs were 1.55 billion as of September 30, 2015, an increase of 14% year-over-year.
Mobile MAUs - Mobile MAUs were 1.39 billion as of September 30, 2015, an increase of 23% year-over-year.
Third Quarter 2015 Other Financial Highlights
Mobile advertising revenue - Mobile advertising revenue represented approximately 78% of advertising revenue for the third quarter of 2015, up from 66% of advertising revenue in the third quarter of 2014.
Capital expenditures - Capital expenditures for the third quarter of 2015 were $780 million.
Cash and cash equivalents and marketable securities - Cash and cash equivalents and marketable securities were $15.83 billion at the end of the third quarter of 2015.
Free cash flow - Free cash flow for the third quarter of 2015 was $1.41 billion."
Source:  Facebook Q3 2015 Results, released 4th November 2015
Note - Full charts deck here