I hear different numbers for this, but the most recent I've come across is $20B. This $20B number is a number I've heard associated with the overall recruitment market in the US. Some percentage of this is online and some percentage is offline. I'm not exactly sure how this breaks (I am quite sure the majority is stillf offline) down between online and offline. Suffice to say that the online segment is growing much faster and continues to take market share away from the offline segment. That is what is creating opportunities for lots of companies in this area (e.g. Monster, Doostang, HotJobs, CareerBuilder, JobFox, etc.). I think the key to being successful online in this space is to go beyond being a digital version of the offline recruitment services. Companies will have to continue to innovate and leverage the power of the web. It will be interesting to see how it plays out.

This insightful article may help:
http://findarticles.com/p/articles/mi_m0EIN/is_2005_July_22/ai_n14813747?tag=content;col1

Since 2002, the online recruitment market has seen significant changes. The economic downturn of 2001-03 markedly affected both revenue streams and the supply structure of the industry. In 2002, for instance, revenues actually fell by nearly 20%, while at the same time record numbers of job seekers were posting resumes to recruitment websites. This began to turn around in 2003, and revenues for 2003 and 2004 are at or above 2001 levels. The revenue outlook is good, as more and more companies and job seekers move their career advertising online. According to industry experts, 96% of Fortune 500 companies now use some sort of online recruitment process. Industry compound annual growth rate through 2008 is expected to be at around 22%.

The poor economy drove many recruitment sites into bankruptcy or out of business altogether, creating an opportunity for larger players to acquire or affiliate with the remainder. The outcome has been the emergence of an industry dominated by three major players: Monster, CareerBuilder and Yahoo! HotJobs, which together account for about 55% of revenues. The remainder is accounted for by a plethora of smaller, industry- or sector-specific sites that appeal to a specialized clientele--the big three are thus actually losing some ground to these niche sites.

The economy is showing signs of a recovery, though job growth remains slower the corporate growth and unemployment has remained at a relatively fixed rate over the past year. Online recruitment, however, is positioned to grow more quickly than other economic sectors. Jupiter Research has predicted that online advertising overall will exceed $16 billion by 2008, adding more than $10 billion from the industry’s total revenues today. Recruitment advertising will likely be the single largest segment of this sector.