Mobile ad revenue is 81% of Twitter's total ad revenue

"Revenue – Revenue for the second quarter of 2014 totaled $312 million, an increase of 124% compared to $139 million in the same period last year.
Advertising revenue totaled $277 million, an increase of 129% year-over-year.
Mobile advertising revenue was 81% of total advertising revenue.
Data licensing and other revenue totaled $35 million, an increase of 90% year-over-year.
International revenue totalled $102 million, an increase of 168% year-over-year.
International revenue was 33% of total revenue."
Source:  Twitter's Q2 2014 Results, 29th July 2014