TheLadders.com is moving from free job postings to subscription fees. HotOrNot just reversed a highly public experiment in which they switched from paid personals to free. Even hippy-dippy Craig's List is steadily abandoning free classifieds in their largest markets, and recently started charging a modest fee in even more cities.While Dave's examples are interesting, I'm unconvinced that they have any bearing on the larger situation. Even if we do see a large-scale return to a paid model, a situation where "modest fees are a form of quality control" as Dave puts it is very different from one in which high prices (relative to cost) were a primary source of margin for otherwise sketchy businesses.
Publishing classifieds in newspapers never cost much money, but given their relative monopoly position, they were able to get away with charging premium prices. Today, the cost of publishing is close to zero, and getting easier every day, ensuring that competition from free providers will be a permanent feature.
As to quality control, all of the larger job boards have long gotten away with charging newspaper-sized prices while delivering content to users that is only slightly less relevant than an Oscar Mayer bacon billboard next to an orthodox synagogue. Bob Wilson continues to document this failure with mind-numbing consistency. Preventing blatant spam and scams is well and good, but it's hardly a major achievement in terms of the user experience, and price is simply the easiest way to do this.
But price is also an obstacle to adoption, one TheLadders used very effectively to build the very brand and audience that allows them to start charging meaningful prices. Prices are sort of like taxes--they always go up, while the value you get out of them seems to go down. Once you get used to charging $25, it's very easy to get used to charging $50, and before you know it, your customers have a good reason to spend the time looking at free solutions again.