The Wall Street Journal writes “Twitter’s mobile ads begin to click.” My GigaOM colleague Mathew Ingram’s headline is “Has Twitter done with mobile ads what Google and Facebook can’t?” And then there’s this. Yes, Twitter is rapidly becoming a mobile-first application. And its in-stream web ads run on the mobile screen. (Facebook’s will, too, and it has a $3 billion web ad business.) But no one actually knows what Twitter’s ad revenue is – regardless of whether it’s “over half mobile” on some days. EMarketer – that missed Facebook’s number by hundreds of millions of dollars before it went public – thinks Twitter sold $140 million worth of advertising last year. It could easily double that and still be chump change. Google’s probably got a billion dollar mobile search business (it has claimed to be on a $2.5 billion runrate), and Pandora is likely the second biggest mobile ad seller, with $100 million.