[bdfh42]

I think you have already identified the answer. It is based upon the needs or disposition of the site visitors/users.

Stackoverflow has a self qualifying audience that is ready to be offered a job/employee matching service.

You have to find that same match for any content site that needs to generate anything more than enough income to run itself - when vanilla advertising will probably meet the need.

I have a similar issue with a site I am working on - it is not yet clear how the audience demographic will settle down. Once it starts to jell then we can probably work from there. In the mean time we are adding some value added services that can be charged for.