From a recent AdAge Article…

…Mr. Bradford (CEO of Vitrue)explained: “If a site has 100 fans and your wall post gets five clicks, that’s a 5% CTR. But if you assume only about 20% of those folks actually saw the post, it’s really a 20% click-through rate.” That’s better than the click-through rate of the average e-mail campaign, he said, and certainly better than the rate for an online ad. It also doesn’t count how many people commented on the post or said they liked it but didn’t click through.

Pasted from <http://adage.com/digitalnext/article?article_id=138442>
 

The question still stands – how many people actually saw the wall post. If a tree falls in the forest does it make a sound?