Job search engine Indeed.com officially became the largest jobs site in the U.S. by passing longtime leader Monster.com in October, according to the latest data from comScore.

A total of 12.3 million users visited Indeed in October, a monthly increase of 19.6 percent. Monster had 12.1 million visitors while CareerBuilder was third with 11.3 million. The numbers, however, reflect the sites’ main jobs sites only and not their entire networks – in which case larger properties Monster and CareerBuilder would still have the edge.

But Indeed has steadily built Web traffic by adding nearly one million new jobs to its index each week, and it could very well become the consistently popular choice for U.S. job-seekers. Indeed actually attracts more monthly pageviews devoted strictly to job searches than Monster, CareerBuilder and HotJobs combined, but the other sites offer additional content such as resume-building tips and career advice.

Monthly traffic to HotJobs, which was purchased by Monster from Yahoo! in early 2010, has fallen 67 percent in the past year.