Starbucks has 4 million mobile app users, and generates 4 million mobile payments a week

"Coffee company Starbucks continues to see significant traction in its mobile efforts--the company said more than 10 million people are "active" users of the company's mobile apps, and that the company now counts close to 4 million mobile payment transactions per week (up from 2 million near the end of last year and 3 million a month ago). This, the company said, accounts for roughly 10 percent of the company's total "U.S. tender.""

Source:  Fierce Mobile Content, 29th April 2013

Earlier - 3m mobile transactions reported in mid March