The premise behind the #3 vs #7 question is flawed. Radio listening is NOT a zero sum game and therefore it is inaccurate to assume that 100% of the WPTF Limbaugh audience would follow Limbaugh to another station - which would, of course, explain why WPTF didn't drop off the face of the earth.
One of the first things you learn about radio is that change in one station causes listeners to look around and see what else is available. Some might actually like Ramsey. Some may have found WKIX. Some may have just turned the radio off.
Since Arbitron doesn't continually consult the same listeners for long periods of time, there is no real way to track their movements from one station to another.
The only conclusive way to answer the question is to do a study of people who listened to RUSH on WPTF and ask them what were their actions when the show moved to WRDU. Doubt that anyone will spend the money on that one.