The San Francisco-based company, run by Twitter CEO Jack Dorsey, issued earnings-per-share guidance for the first quarter between 6 cents and 8 cents. Wall Street was looking for 11 cents. Its revenue guidance for the first quarter was in line with analysts' expectations.
Slower growth also weighed on Square's stock price. The company's adjusted revenue grew 53 percent year over year, excluding acquisitions — down from 56 percent in the third quarter.
Subscription- and services-based revenue, a metric analysts watch closely, was a bright spot. The company reported $194 million in that portion of revenue, a 144 percent jump year over year. For the full year, Square notched $592 million in subscription- and services-based revenue, up 134 percent year over year.
Square's peer-to-peer Cash App had another breakout quarter, with more than 15 million monthly active customers in December 2018, doubling from a year earlier.
Dorsey told CNBC in a statement that more than half of Square's adjusted revenue came from products launched in the last five years. A "network effect" of friends and families helped growth in downloads, he later said on a call with analysts.
"People are utilizing the money they have in Cash App with friends, families and landlords, causing another download, and another, into the network," Dorsey said.
The payments company announced it hired Amrita Ahuja as its new chief financial officer in January. Wednesday marks her first earnings call in the position, after former CFO Sarah Friar stepped down last year. Ahuja held the same position at Blizzard Entertainment, a division of Blizzard.
On the earnings call, Dorsey highlighted Square's debit card for businesses that launched in January. It gives Square merchants immediate access to sales made on the payment system, and it added to the company's growing suite of banking products. The card enables someone to start a business "without even having to go to a bank," which Dorsey called "pretty powerful, pretty profound and really cool."
"Square Card has been another highlight over the past three months," he said. "This is a way for us to continue to serve underserved and unbanked sellers."
Dorsey said 40 percent of merchants didn't have a business debit card before using the Square Card. The company has increasingly moved into traditional banks' turf, mostly by servicing small businesses. It also issues loans through Square Capital and refiled its application for a bank charter in December. An approval would allow it to take federally insured customer deposits.
"For the year ahead we continue to focus on three things and we will continue to strengthen our omni-channel offering," Dorsey said. "That means that we add strength to in-person payments, to mobile payments and also to online. We're really excited about everything we're doing in financial services."
Square launched bitcoin trading through the Cash App in January 2018, and generated $52 million in revenue from the cryptocurrency in the fourth quarter — up from $43 million in the prior quarter. For the full year, the company made $167 million in bitcoin revenue.
The stock has rallied 40 percent this year, and more than 70 percent year over year. Square shares were slightly higher ahead of the earnings announcement, trading near $78.95.
"While 4Q was not something to write home about, the SQ story is likely far from over as new products and services can provide a second wind to growth as the year progresses," Nomura Instinet analyst Dan Dolev said in a note to clients Wednesday. "We expect a negative stock reaction given somewhat underwhelming results."
— CNBC's Deidre Bosa contributed reporting.
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