Global payment firm MasterCard ( MA) knows a thing or two about the network effect too. The firm operates the second-biggest payment card network in the world, and owns a handful of brands (such as Maestro and Cirrus) that help consumers pay for products and services more easily. As with shipping, the payment network business has high barriers to entry, and while there are more players in the industry, MasterCard's 31% share of the market speaks to its concentration (along with Visa (V), the firm claims more than 90% of the payment card industry). MasterCard benefits from the gradual shift away from cash and checks and towards electronic payments. So even though MA is the No. 2 player in the industry, rising tides stand to lift all ships as more and more consumers shift spending onto cards. And because MasterCard is the network, not the lender, the firm's balance sheet is free from exposure to credit risks that could send banks reeling if defaults started climbing. Spending is the only thing that's tied to MasterCard's financial performance. Back to that network effect. Because MasterCard is accepted almost universally, it's a card that consumers want to carry. And in turn, it's a card that more merchants prioritize accepting. The firm's huge installed base should keep its spot secure for the foreseeable future. As of the most recently reported period, MasterCard was one of Warren Buffett's stocks. To see all of this week's Rocket Stocks in action, check out the Rocket Stocks portfolio at Stockpickr. -- Written by Jonas Elmerraji in Baltimore.