Square, Google Wallet, Isis and plenty others are all working to grab the lion's share of the growing (but nascent) mobile payments market. But, so far, none of these options are as widely used in North America as Starbucks' simple mobile apps, according to new findings from the research firm Berg Insight. Across the continent, Berg claims that about $500 million worth of mobile payment transactions took place last year. "However, the vast majority of these payments were made using Starbucks' phenomenally successful smartphone app, whereas mobile wallets that can be used at multiple merchants have yet to gain traction," the research firm said.

Unlike mobile wallet apps, Starbucks' app works essentially as a digital gift card that can be reloaded and used only at the ubiquitous coffee chain. "In the longer term, universal mobile wallets such as those provided by Isis, Google and MCX will drive the majority of the mobile in-store purchase volume," the firm said, adding that it projects annual mobile payments will hit $44 billion by 2017 as services are more widely accessible and grow easier to use. It's going to take some effort to get there, though: Google Wallet has seen plenty of resistance from carriers, and Isis remains live in just a small handful of markets.