Cost of advertising.
The cost of some of the job boards is getting ridiculous with the £1,000 ad already here if you want the ad, plus a logo/highlighted listing plus a small button logo on the homepage. And, if it doesn't work we are getting ever closer to print; high cost no result!!
Autotrader charge just £15 for a 2 week advert if you are selling your car. Of course they offer enhanced listings, but for £15 it's a no brainer and if it doesn't work, it's not the end of the world and maybe you look to improve your listing. But if it was £100+ just to try it out, you'd think twice and would be pretty pissed off if it delivered zilch.
So, as job boards and search engines get more traffic, up goes the cost of getting your ad noticed, out come new places that are less cluttered, lower cost and work like the boards did before too many people went to them!
Ebay jobs, Amazon jobs, Gmail jobs, Skype jobs - who knows where else we'll see them.