I do have a question about impressions that I don't think has been addressed: do unique users have a "duration"?
Say I have some loyal follower who visits my page every day. I'd get an impression the first time they visit, but what about the next days? Will I never receive another impression from their IP or (however it tracks it) ever, ever again (for that particular page)?
In some ways it makes sense to limit it, but it also makes sense to put an expiry/duration on something like that. After all, mere exposure is a powerful force: ad companies should want the same person to be exposed as much as possible to an ad!