When I see advertisement on WBB, and calculate number of impressions, I see same shit, making money from traffic, that is connected to pirated material. It doesn't matter if owners don't upload. They use uploaders to attract others, and money is rolling in. They have decided to cut off other links, because they start to lose profit, no one is interested in paying for premium account on other service than rapidshare. Since rapidshare links attract people, this directly turns into traffic, which turns into money.
They don't just pay servers from ads. They make quite a lot on the top of it.

And, they don't care if number of uploaders gets cut in half, because number of downloaders will fill that gap and add some extra, and everyone is happy. See how they care about their users?