Perhaps a more reliable measurement of visibility is the number of visits made to their websites through signatures, or perhaps the number of users referred by signature links.
True, but that's not the kind of information a random person like the OP would have. They would have to contact the casinos and ask for the information from them, and I don't know if they would be willing to share it in public. It also needs to be done regularly as each week/month can have different results.
I don't think that many people click on random signatures anyways. Perhaps only on those from high-quality members that are instantly recognizable.