With Tailored Audiences, Twitter has signaled to the rest of the ad world that it truly is open for business. The new advertising service opens Twitter’s mobile app for retargeted Sponsored Tweets to users based on their Web-surfing habits on desktop. To introduce the capability, Twitter worked with data firms that work with brands, and in some cases, these were the first major partnerships for Twitter in this world. For example, AdRoll, with close ties to Google and Facebook, is now working with Twitter to introduce its clients—among them, Salesforce, New Relic, Alex and Ani&...