Twitter took $665 million in ad revenue in 2013

"Total ad revenue generated by Twitter increased 110% to $664.9m in the year to 31 December.
Earnings before interest, taxes, depreciation and amortisation grew 256% year-on-year to $75m. Operating expenses such as marketing and research and development meant Twitter posted a full-year net loss of $645m.
In the UK, Twitter generated $23.5 million (£14.1m) in the last three months, and $43.1m (£25.7m) in the previous nine months.
However, Twitter's average ad rate has been declining since 2012. It’s down 81% from that point, according to data compiled by Quartz from regulatory filings.
Twitter concedes rates have been falling because its inventory—the supply of available space to advertise on Twitter—is increasing. It also says that lower rates appeal to small businesses and international advertisers."

Source:  Campaign, 7th March 2014