Square Inc., the mobile-payment startup, said Chief Operating Officer Keith Rabois has resigned, marking the departure of an executive who has helped the company grow to more than 3 million customers.

Chief Financial Officer Sarah Friar will become acting COO, San Francisco-based Square said yesterday in a statement. Friar joined the startup in June after a stint at Salesforce.com Inc. Rabois, who previously worked at PayPal Inc. and LinkedIn Corp., joined Square in 2010 to help with marketing and business development.

When Rabois started, the company had 17 employees, and fewer than 1,000 merchants were actively using the product -- a stamp-sized device that enables credit-card payments via mobile phones. Valued at $3.25 billion in August, it now has more than 3 million individual and business customers and is processing more than $10 billion in transactions on an annual basis.

“Leading our amazing crew has been the most rewarding professional journey of my life,” Rabois said in the statement. “But every day matters. And it is better at this point for me to be doing something different every day.”

Friar joined Square in July following a year at Salesforce.com. Prior to that that she spent more than a decade in the technology group at Goldman Sachs Group Inc.

In addition to his operating roles, Rabois has been a prolific angel investor, having backed more than 35 companies including YouTube Inc., Yammer Inc., Airbnb Inc. and Pinterest Inc., according to his website. He’s also on the board of review site Yelp Inc., which went public last year.

In December, Square stepped up competition with PayPal by letting small merchants sell gift cards to shoppers on the go. The two companies are competing for business in the mobile payments market, which Juniper Research estimates may top $170 billion in transactions by 2015, up from $60 billion last year.

“We couldn’t have done it without him and we wish him well in his next opportunity,” Dorsey said in the statement.

Starbucks Corp. contributed to the $200 million in financing in August that gave Square its valuation.

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