When listening to Sergey talk about Google Buzz ten minutes into this video he brings up the value of using buzz to ask questions to his contacts with the recommendation system directing the question to who best can answer it.
Pagerank doesnt have anything to do with twitter and facebook.
Its a great idea, and its probably one of the major factors that made google a success, but today it is not a very important factor in google being a bigger company than fb and twitter.
Perhaps I poorly worded by saying "ten minutes in", I meant starting from 10 minute and 10 second mark Sergey brings up how he's been using the service internally.
Edit:But going back to your point, I not sure Pagerank is relevant to twitter/facebook, and I don't think Sergey was saying that it was. But I do think their could be search problem emerging in social networks when people start having several hundred friends and you want to find relevant conversations to your interests going on within your network.
http://www.youtube.com/watch?v=d_W6Qbob2mg#t=10m10s
I was like "this sounds like Aardvark".