Those are some tough questions and if there was a straight forward answer FB would probably give it. The "ad performance" part for CPM ads comes from how people interact with the ad and if they like it. They use complaints and ctr as well. Lately this has been starting to level off much better with more even CPM spending, before I think it was more to do with there system messing up then correct "ad performance" for CPM ads.
There is a very good reason FB has not published a frequency cap or given us that option that I know of. They still play around with it themselves quite a bit. But one thing is for certain, CPM ads are seen by the same people more often per day then CPC ads on average.
Although they would like to do this it simply isn't possible given the volume of ads they have to serve. Also it's very easy to investigate this idea if you are curious. You can actually see where people came from in FB based on the refer: photos, homepage or even what app they were playing. Run 2 ads with CPC vs CPM and compare the refer info, in my experiments it matched up pretty evenly once you are past 1000 clicks.
BTW my research is based on talking to them(not very useful) spending money and also a nice little web of accounts I have all with a unique keyword so I can target just those accounts and see the ads first hand and play around with the system.
God damn I hate Facebook, my media buys better go nicely in 2010 as I'm tired of messing around it.
ENJOY
