I've seen a few sites with an ad that pops up when you start to leave, and that ad is almost always a multiplex. Which makes sense, since that's a high paying ad unit right now.
But at least half the time, when I show a multiplex on my site, it only has 1 little unit in it! I have a large spot set aside that doesn't even collapse to fit >:-(
Has anyone found a way to test how many ad units will show up in the multiplex BEFORE it's displayed? So that if, for example, it only has 1 ad unit, then I could skip it and show something else?
Failing that, any thoughts on how to make the multiplex compete with a responsive display banner so that it always shows the highest paying ad?