You get feedback loops.

If I mention emu farming in this thread, you can bet Google is going to give somebody an ad offering emu chicks for sale, or emu egg hatchers, or emu-proof barbed wire, or emu caponizing pincers. Then somebody comments "hey, why am I getting adverts about emus?", and that triggers even more adverts about emus, emu marketing software, software for blocking ads about emus, and so on. In short order your whole screen is emus, with bits of message here and there, buried under piles of moulted emu feathers and emu droppings.