What really makes me wonder, is who the hell clicks on signatures? Pretty much nobody I'd think, but apparently that can't be so.
A small percentage of the thousands of people that read every post click on the signatures. When you multiply that by all people wearing that signature and all the posts, it makes a significant amount.
Apart from that, even if people dont click on the signatures, they are exposed to advertisements and some of them will be more likely to use that service in the future.