Consider the difference between a Facebook profile and a TripAdvisor travel review. A typical [Facebook] pageview ... is by someone known very well to the creator of the profile – a close friend or acquaintance ... A TripAdvisor travel review is seen by people completely unrelated in any way to the person or persons who wrote the reviews on the page.When a page draws from a wide audience, the people who manage to get to that page often will come from search and will be interested in the content of that page. So, they can easily be targeted by targeting the content of the page.
The "affinity" of a social media service is the average closeness of relationship between a content creator and someone who views that content. The affinity of Facebook is very high, while the affinity of TripAdvisor is very low.
There is an inverse relationship between the affinity of a social media service and its targetability. Why is this true? The act of viewing a Facebook profile gives us very little information about the viewer, other than the fact that she is friends with the profile creator; when someone views a TripAdvisor travel review, she is definitely interested in traveling to that location.
When a page draws from a narrow audience, like a group of friends, those people are not as interested in the content as the person. So, targeting the content, as contextual advertising does, will be less effective than targeting the people.
I have said before that lack of commercial intent is the problem on social networking sites, but Anand makes the good point that diversity of interest may be a better way of describing the problem. By summarizing why people come to a page, affinity may explain the difficulty of targeting advertising to that page.