My adsense link unit coverage is low. A lot lower than the coverage for the other adsense ad units so I tried to figure out what is going on.Google has this to say:
For link units, coverage reflects the number of link units that showed ads after the user clicked on a term in the link unit. Coverage for pages displaying link units is typically much lower than for the other kinds of ad units, because it involves user clicking. This also means you’ll see a higher number for unmatched ad requests.
Well, that is not very concrete at all. What they means is:
(how coverage is defined for all other adsense ads)
So if you assume that the actual coverage of your adsense link units is about the same as the coverage of all your other adsense ads you can easily calculate how often visitors clicked on adsense link units.
This is important to know because you can see how often users only clicked once, but did not click on any ads after that. If this happens too often for some adsense link units think about replacing them with actual ad units to increase revenue. Although expect a slight drop in clicks when doing this as users tend to click more often on adsense link units than on ad units.