This week Twitter's CEO, Dick Costolo, announced that the company's mobile ad sales have been exceeding web sales. - 

Facebook is still trading almost 30 percent below the price it went public at a few weeks ago. Investors still aren't convinced that the company has figured out a way to make money, especially on mobile phones. But this week, Twitter's CEO said their mobile ads are booming, already outpacing web ads and on track to keep growing.

Peter Kafka writes for All Things Digital and has been covering this mobile ad puzzle. He joined us for out mid-day extra to explain why Twitter succeeds on mobile while Facebook is stumped.

Follow Jeremy Hobson at @jeremyhobson