Twitter CEO Dick Costolo revealed at San Francisco's Web 2.0 Summit on Monday that Twitter's deep integration in iOS 5 seems to be paying off. In iOS 5, Apple weaved tweeting capabilities into native apps ranging from the photo app to Safari, and gives users the ability to sign in or sign up to Twitter from the Settings app — no download required. Since iOS landed on October 12th, signups for Twitter via iOS devices have tripled, which Costolo notes is even better than expected, Electronista reports. User engagement is up too, with over half of Twitter's users logging in at least once a day, up from 30 percent in January, and over 250 million tweets being sent daily. With Apple reporting that 25 million users have already upgraded to iOS 5, it's no surprise that sign up rates are rising, but Twitter will need more than that to reach Costolo's stated goal of 2 billion devices.