economics of Pay Per Download?

I understand how Pay Per Download, etc., work from the affiliate's perspective. What I don't understand is how the file hosting sites, like filesonic, make money. What are they doing that allows them to profit by my having people download files from their site?

I've looked here and elsewhere, but haven't found an explanation. I have this feeling that it's obvious to everyone but me, so I'm sure I'll feel like a d*mb*ss once someone explains it.

FYI, I'm not contemplating starting such a service, this is just something that's been bugging me a long time....

Thanks much.

it's the advertisements and if there are none (which I highly doubt on free download servers especially), it's probably to see the inpouring of traffic

advertising and subscriptions