San Francisco-based Affinity operates seven professional and career-oriented networks for professions that include law enforcement, health care and education. Monster said the network will provide its users with career guides, industry job searches, trade news and social networking while allowing the company to increase its online advertising services to a larger customer base.

The acquisition comes less than a year after Monster and Affinity signed a deal that allows job seekers to access Affinity’s Web sites when registering for a MyMonster account. Affinity Labs founder Christopher Michel also founded Military.com, a site for military personnel and their families that was acquired by Monster in 2004. Manhattan-based Monster said it will keep Michel on board after the acquisition is complete.

Earlier in the week, Monster announced “Your Calling is Calling,” a global branding campaign aimed at marketing Monster as a way to find one’s “dream job,” instead of just a source for job listings. The new campaign, crafted by Manhattan-based BBDO Worldwide, includes television, radio, print and online ads.

Shares of Monster fell as much as 6 percent to a 52-week low of $27.19 Friday, and were down 4.8 percent in afternoon trading. Shares have traded between $28.55 and $54.79 in the last 12 months.

Late last year, Monster’s board authorized a $350 million stock buyback program, shortly after eliminating 800 jobs, or 15 percent of its 5,500 full-time workforce.

Filed by Kira Bindrim of Crain’s New York Business, a sister publication of Workforce Management. To comment, please e-mail editors@workforce.com.