David Beisel discusses four categories:
Behavioral-based advertising networks matching targeted ads to people based on who they are rather than page context. An advertisement is effective if it is relevant to a specific person, regardless if that ad isnt matched to the current page (which is difficult to discern contextually in many social sites given the nature of the content).
Syndication advertising widgets facilitating the tools to spread marketing messages in a distributed web. The current use-cases of branded badges, contests, and music video promotion are just the beginning of widget advertising.
Social shopping/commerce providing consumers with rich social context and relevancy to the purchases which they are making with inherently monetizeable content.
Integrated mobile platforms incorporating medium with embedded billing mechanism. While consumers are reluctant and to pay for services on the web, theyve repeatedly shown a willingness to pay for content on their mobile (ringtones, premium SMS, etc.). There is further opportunity to marry these two worlds working towards the goal of monetizing the tremendous traffic online.