Dave Beisel has a thought-provoking post on Social Advertising.  He sees it like this:

"I am not referring to the notion of marketing with a goal towards positive societal change, but rather the concept that relevant
online advertising could be potentially delivered to consumers based on
information about which ads were well received by those in their social
network? For example, if I click on an ad about, say a
specific car, it’s likely that those who I know are also in a
demo/psycho-graphic would also be interested in the same advertising
content."

I agree with him that the offering would be valuable.  However, many social networks (or sites that provide connections through  shared friends or interests) offer this in an implicit manner.  For example, MySpace provides a platform for groups and Green Day has a MySpace group with 121K members.  Group members, many of whom are members of bands themselves post on the group to expose themselves to Green Day fans.  The same phenomenon can be observed on Yonja.com, the 2nd largest Turkish web property, where brands create associations with each others’ "fans" through a groups functionality.  This is a version of what Beisel’s thinking of.

Social listing sites like Tribe.com and Judy’s Book,  offer social advertising through a different class of ads.

And finally, Seth Goldstein’s /Root Markets is trying to create a marketplace of attention, which, at one point, will (should) incorporate a social aspect and connect preferences and associations.

As content gets microchunked, so will ways to track its consumption and monetize it.  Welcome to Media2.0.