Last year, employers filled more than half of job openings with existing employees, a new study shows.

Internal transfers and promotions accounted for an average 51% of all full-time positions filled in 2009, up from 39% in 2008 and 34% in 2007, according to CareerXroads, a staffing-strategy consulting firm in Kendall Park, N.J. CareerXroads surveyed 41 companies that employ a combined 1.8 million U.S. workers. Last year, these firms collectively filled 176,420 positions.

For the 49% of jobs that were filled with external recruits, referrals accounted for the most hires -- 27% -- about the same as in 2008. On average, these yielded one hire for every 15 referrals received. Company Web sites accounted for 22% of external hires and online job boards 13%, according to the survey.

Mark Mehler, co-founder of CareerXroads, says the findings indicate that networking is the most effective strategy for landing employment. "Job seekers should use job boards and corporate sites to find information about openings, but they should use their network to apply."

Among the job boards that respondents credited for netting outside talent, CareerBuilder.com came out on top, accounting for 42% of external hires -- however one company in the survey claimed a significant portion of these. Monster.com netted 12% of external hires. Aggregate job sites, which advertise openings from multiple job boards, hooked up 10%. Classifieds site Craigslist.org accounted for 2.8%.