All of the sites can tout number of members, number of visitors, number of ads, number of partnerships and on and on. What none of them will discuss is the success rate. The same ad will frequently appear on multiple sites, so it's almost impossible to know whose ad worked. Plus, it doesn't seem like any of these employment Web sites are really interested in the job seeker really getting the job. The focus is on the revenue from the job posters. These sites also make money from side efforts like resumes, coaching and advertising.
It is rare to find any job Web site that promotes itself as successful for job seekers. Taleo claimed that it was responsible for 9,000,000 hires in 2010, but there is little information to back up that claim. That, however, hasn't stopped the IPO market from convincing investors that the LinkedIn model can be replicated or that once public you can get bought out by a big whale like Oracle. PDN doubled the size of its offering, even though it clearly states in its filing that it lost its biggest customer and the replacement will generate at best half the revenue.
Ultimately, the investor in PDN will feel as much success as an online job seeker - time and money wasted.