An interview with patent guru Bill Slawski had this little gem stuck at the end of a segment concerning search engines producing queries that are more social nature:
As far as optimizing a site when search engines may be considering user behavior more, it still helps to know something about the audience for your site, what they are interested in, and what words they will likely choose to find your site.
That’s always the bottom line, isn’t it? If your site is about weaving baskets and you know that your users are more interested in hand-made wicker baskets than Longaberger then it is important to optimize your site for “hand-made wicker” and related terms and not for “Longaberger”. Searchers will use the terms that are related to their interests. Your job is to use the terms on your site that match those terms. You can’t get any more “user behavior” than that.