I think FreeBitco.in, could technically generate a lot from advertisement, considering their traffic. But they have opted for no ads, and rightly so. Annoying ads usually scare off the users. The lower the user base, the less companies are willing to pay for ads, and thus advertisement income dries up. Basically, it's the road to bankruptcy, and it's good that FreeBitco.in didn't step on this road of no return.