What makes you think that? They have a big site, without any advertising banners...doesn't sound like a very commercially attractive deal to me.
[edit] Oops, my bad, they do have some banners, my ad blocking tools turned out to be far more effective than I thought they would be. Even if they serve those banners, I really can't imagine they are making "big money". [/edit]
Im sure they dont make a lot of money out of it though (running costs perhaps)
EDIT: @jvdh - heh yeh I only happened to notice because I was using IE (spit) at the time.
This is just quessing, i have no confirmed information.
For a group that list openness and freedom amongst their ideals [sic] I dont see any openness about the revenue TPB generates. They release no figures or data.
Personally I consider that a double standard (and I have to confess I do believe they make a profit: perhaps not a huge amount but... logically they could make quite a bit and I dont see a decent argument for why they wouldnt)