Mobile accounts for more than 75% of Twitter's ad revenue

"Average Monthly Active Users (MAUs) were 241 million as of December, 31, 2013, an increase of 30% year-over-year.
Mobile MAUs reached 184 million in the fourth quarter of 2013, an increase of 37% year-over-year, representing 76% of total MAUs.
Timeline views reached 148 billion in the fourth quarter of 2013, an increase of 26% year-over-year.
Advertising revenue per thousand timeline views reached $1.49 in the fourth quarter of 2013, an increase of 76% year-over-year.
[...]
Revenue - Revenue for the fourth quarter of 2013 totaled $243 million, an increase of 116% compared to $112 million in the same period last year.
Advertising revenue totaled $220 million, an increase of 121% year-over-year.
Mobile advertising revenue was more than 75% of total advertising revenue.
Data licensing and other revenue totaled $23 million, an increase of 80% year-over-year.
International revenue totaled $66 million, an increase of 200% year-over-year. International revenue was 27% of total revenue."
Source:  Twitter's Q4 2013 Results, 5th February 2014