By John Letzing Of DOW JONES NEWSWIRES SAN FRANCISCO -(Dow Jones)- Amazon.com Inc. (AMZN) has set a torrid hiring pace in recent months, accelerating a rate of employee acquisitions that was already well ahead of most large corporations. The sharp influx could leave the Internet retail giant well positioned for future opportunities--or possibly overextended, analysts say. Amazon recently reported that it loaded up on nearly 28,000 new employees in the past year or so, a 73% increase that brought its total headcount to 65,600 as of the end of March. That annual increase is the largest the Seattle-based company has reported stretching back more than a decade. It also capped what has been a breathtaking run of hiring over the span of the past few years. Amazon's more-than-three-fold increase in headcount between 2007 and last year placed it sixth among companies in the S&P 500 index in terms of employee growth rate. Some companies ahead of Amazon on the list, including telecom firm CenturyLink Inc. (CTL), expanded in part thanks to large, multibillion-dollar acquisitions. Amazon paid roughly $1.2 billion during the period for Web shoe retailer Zappos.com Inc., which had about 1,600 employees at the time. As the recession faded, Amazon's hiring maintained a healthy pace with revenue growth; sales per employee rose to more than $1 million in 2010, from roughly $873,000 in 2007. But the measure slipped back to about $856,000 per employee last year. It is unclear how many of the employees Amazon includes in its headcount figures are part time. In any case, the company's hiring raises the question of whether the company is responsibly preparing for expansion--or potentially endangering profit margins that have already drawn some concern on Wall Street. "Amazon without question has been getting a pass in terms of some of their costs and some of their investments, at least to date," said Dan Geiman, an analyst covering the company for McAdams Wright Ragen. In the first quarter of this year, Amazon's operating margin stood at 1.5%, compared to 3.3% in the period a year earlier. Stifel Nicolaus analyst Jordan Rohan said the hiring pace could be seen as an encouraging sign of confidence--though investors won't necessarily forgive much more erosion of margins. "I can see the glass half full, or half empty," he said. An Amazon spokesman declined to comment. Much of Amazon's hiring is happening at its bustling shipping centers. The company opened 17 facilities last year, and says it plans about 13 more this year--which would bring its total to 82. But Amazon is also moving into a dizzying array of businesses ranging from original TV programs to high-fashion retail, with more uncertain futures than its core business of selling and shipping goods. "They're obviously just moving in every direction," Geiman says. Some of the company's recent job postings reflect the diversity of its needs: A software engineer to work at Amazon's Wisconsin-based women's apparel site Shopbop.com; a marketing specialist to expand its gift card business in Germany; a developer at the firm's Silicon Valley facility Lab126 to help design Kindle devices; a lawyer to oversee movie licensing for its London-based Netflix Inc. (NFLX) competitor LOVEFiLM; and a medic at a Lebanon, Tenn., shipping center. Amazon has also advertised for a leader of a new laboratory dedicated to developing "previously untested methods" of recruiting top talent. The group would focus on "understanding the candidate's life trajectory rather than their domain expertise," Amazon said in a posting that was recently withdrawn from its website. During a conference call with analysts following its earnings report last month, Amazon executives said much of the hiring done in the first quarter was in "operations and customer service." The company also added some temp workers used during the holiday shopping season to its workforce during the period, they said. "We'll moderate it at some point," Chief Financial Officer Tom Szkutak said of Amazon's hiring pace during the call. But he added: "given the growth that we're experiencing right now, we are investing and we're making sure we have the right resources." -By John Letzing, Dow Jones Newswires; 415-765-8230; firstname.lastname@example.org --Scott Thurm contributed to this article.