How does Some Websites generate Revenue?

Digg, Wikipedia revenue generation

[knowdeal]

This question may be naive. Or may be with an obvious answer. But I think somehow it's not the whole truth.
Kindly correct me if I am wrong or mistaken. Also Please bear if this is not the pertinent forum to post this.

How does some websites generate revenue? Especially the ones like Wikipedia, Digg.com, Popurls.com etc. [many many other sites as well] which doesn't seemingly post any Ads [Adsense or Any other Ads] on their webpages? Or doesn't run any Affiliate programs or any other apparent revenue generation schemes on their sites?

How do they make money? What do they do to sustain? I know Digg.com makes so much money. But how? Is it just the Brand Value ie., the value these sites create? Or are there any other hidden things?
What are the parameters?
What are the Different Methods of revenue generation for sites those aren't directly selling anything, not displaying any ads, are without affiliate links? How and Why do they exist?

More Traffic is More Value. But how does traffic matter without Ads running on the pages? So ,how is Traffic converted to [Monetary]Value?

Kindly enlighten. And Please explain with examples where possible.

Thanks.

[willmullis]

Not sure what you are seeing but I see banners at the top of every page on Digg. For WikiPedia I do know they rely mostly on donations.