Square's electronic payment app has helped credit card companies infiltrate small merchant businesses. Farmer's markets, food trucks, and hole-in-the-wall bookstores -- locations where credit cards were practically non-existent -- are now able to accommodate customers with plastic. Square has been a staple of the IPO rumor mill since it hired Sarah Friar as CFO in June. Formerly a vice president at Salesforce.com (CRM), Friar also worked for 10 years at Goldman Sachs (GS, Fortune 500) and is the kind of experienced CFO companies need to make their way through an IPO. Square's partnership with Starbuck (SBUX, Fortune 500)s in August pushed its end-of-year 2013 valuation estimate as high as $5.5 billion.
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