In his talk with Fortune writer Adam Lashinsky (video embedded below), Costolo noted that while Twitter took three years to get to one billion tweets, it now sees that many messages posted to the network every five days. He also said that there are more than 200 million registered users of the service (defining “active users” is difficult, if not impossible, he argued) and that third-party analytics show that the website gets more than 400 million unique visitors a month. Costolo added that Twitter is seeing 40-percent growth every quarter in terms of mobile users. Costolo also spent much of his time talking about where Twitter sees future revenue and profit opportunities — in addition to the ongoing rollout of advertising through “promoted tweets” and “promoted trends,” as well as a forthcoming self-serve ad product (how users respond to these efforts remains to be seen). The Twitter CEO said that the company sees a number of opportunities when it comes to enabling — and taking a share of the revenue from — direct e-commerce with users via the service, because “we already see a tremendous amount of commerce taking place on the platform.” Get the full story at GigaOM