well there were pay per click sponsors before
click-thru calvin i think it was and others
i think per-signup finally killed per click due to how popular it got and there was also lots of click fraud going on.

The pay-per-click programs though had one requirement that you had to convert at a certain rate and if you didn't they switched you to revshare or per-signup. Can't remember who it was but they had something like 1:250 ratio and another sponsor had 1:100 - so it wasn't such easy money .