Instead of showing crappy CPA offers the publisher should show either nothing at all, or some relevant site content. Show a snippet of the friend-feed, or maybe a list of 'online friends'. Show "interesting related links", or "new photos posted"… it doesn’t really matter. Show something that is of interest to the user. The point of the exercise is to train the user to start looking at this specific space again.
If this is obviously so good, why is nobody doing it? Well there’s only one small insignificant problem… Publishers have no way of identifying the top 20% of impressions. You see, especially on social networking sites a huge portion of that 20% are impressions that are sold behaviorally via ad-networks and exchanges. Those ad-networks and exchanges need to see the full 100% to be able to cherry-pick the 10% that are valuable to them thereby making it quite difficult for the publisher to “not show ads” on worthless impressions.
Showing something other than ads when there is no money involved is a great idea. Unfortunately, most traditional ad networks have no interest or capability to do this. Even 'behavioral' targeting folks aren't in a position because they have only a few 'behaviors' rather than a full tagcloud of interests.
Our Others Online affinity profiling system has behaviors, interests and a measure of the commercial value of those interests which means we can power ad units that know when its the right time to show an ad, or whether it's better to show relevant news or other content.