We are already waiting quite a while for Google (or Yahoo) to incorporate social networking technology into their web search services. Personalization of search results (as already being offered -- of course in some limited way) as a very first step towards the right direction has become part of traditional web search. The next step should include not only search results from personal resources but also from resources being available within your personal social network. Seems to be straight forward...?!
First thing, include all resources that I have tagged (you can also distinguish between resources that are yours and resources owned by others that you have tagged). In addition to the traditional hyperlink graph of the web (as it is used by the PageRank algorithm), new (weighted and labelled) arcs have to be inserted connecting your homepage (site/blog or whatever) with all the resources (own and foreign) that have been tagged by you.
Ok, now let's consider that everybody's tag-links are inserted into the hyperlink webgraph. Next thing is to insert (weighted and labelled) arcs from your homepage to the homepages of all your friends (according to your personal social network).
You will end up with a graph that includes (a) traditional hyperlinks, (b) personal tagged + weigthed links to resources, and (c) personal tagged + weigthed links to other users. This composite social webgraph should be sufficient for extending the traditional PageRank algorithm to include social networking information.
Of course (and I'm pretty sure of that and I don't go into details now) a lot of adjustments concerning the weights and the use of tags/labels for indexing have to be considered. But, I think it should be possible...
To some extend, a similar approach has been implemented by lijit. lijit is a personalized search engine that makes use of all your available social networking information. At registration, besides your homepage (or blog ... unfortunately you can not manage several different blogs) you pass over your username for several bookmarking/social networking services as well as the URLs of (a) content provided by you and (b) websites (blogs) of your friends. lijit creates a searchable webgraph covering all the resources and networking information that you have provided. Therefore, searching with lijit comes close to a rather personal variant of searching your very own web-universe.
Social Graph Based Search is also the topic of a video podcast from Scobleizer. Although I can't follow his argument that the social networking companies will kick Google's butt in four years (he states that Google's PageRank algorithm cannot be adapted to include social networking information...at least not in a way scalable for Google's purposes and also not with the current business modell of today's SEOs [=Search Engine Optimizers]), I agree to the way how to include social networking information into traditional web search.