The New York Times Co. swung to a hefty $15 million profit last quarter from a $4 million loss a year ago in part on a one-time gain from the sale of a stake in Fenway Sports Group.Digital advertising revenues at the News Media Group rose 6.2% to $50 million due to growth in retail and national display advertising. The flagship paper went behind a pay wall last spring, and so far the move has proved a success. Total revenues dipped 3.1% to $537 million from $554.3 million. Total ad revs fell 8.8%. Circulation revenues increased 3.4%. CEO Janet Robinson said the news division’s digital subscription initiatives “remained our top focus.” The Times now has more than 324,000 paid digital subscribers and paid and sponsored relationships with over 1.2 million digital users. Those include 100,000 sponsored by Ford Motor Co.’s Lincoln brand who have free access until year’s end and 800,000 home-delivery subscribers. Robinson said the launch of the digital platform has benefited home delivery, with an increase in new orders and improved retention. Digital advertising revenue as a percentage of total company ad revenue was 28.6% from 27.3% a year ago. Total digital advertising, which includes NYTimes.com, BostonGlobe.com, Boston.com, About.com and other sites, saw revenue dip 4.5% to $75 million.