I've been having a similar conversation with a number of folks from the world of search that's interesting enough as to deserve some transparency and discussion. It centers around the idea of the web's link graph and how it operates to power the rankings of relevant results in the major search engines. If we follow this brief timeline, you'll see what I'm getting at:

• 1993 - 2000: The beginning of the web is marked by an influx of researchers, academics, hobbyists and enthusiasts. Nearly every link created has an editorial, reference purpose behind it. A link is one page telling its viewers that another page has useful, interesting or worthwhile information about a specific topic.
• 2001 - 2005: As the web commercializes at an accelerated pace and PageRank becomes a familiar concept, links drift further away from editorial votes and more towards self-interested endorsements, often with financial motivations.
• 2006 - 2010: The web's link graph swings further away from editorial references towards ever-more commercial interests. Meanwhile, the social web rises with the popularity of sites like StumbleUpon, Digg, Reddit, Facebook, Twitter & LinkedIn. These communities often contain a much higher percentage of editorial citations, particularly those that contain smaller communities inside them (LinkedIn groups, pockets of Twitter users and Facebook friends)

During chats with some folks from Bing, Google & the SEO world, it became clear that nearly everyone is aware of this ecosystem and thinking more about how to leverage it to make search better. Bing & Google obviously made back-to-back deals to get the Twitter firehose late last year. Google's been trying hard to get Facebook data without success (and Bing may have it, thanks to their investment in Facebook in 2007). Both engines could certain extract citation data from other web communities that publicly publish (Delicious, Reddit, Digg, LinkedIn, StumbleUpon, StackOverflow and as of today, Quora) and extrapolate reference material.

The problem for the engines is that links on websites have a high probability (probably not 50%, but maybe as high as 20%) of existing specifically to influence their rankings. While some of those influence-targeted links certainly do point to great content that's relevant and high quality, the engines would prefer to return to a web of "pure" recommendations. The social web might offer more of that type of web environment. Sure, we all tweet/share/post links to our own websites, but those are easy for engines to detect and treat as "internal" references. The "external" endorsements, however, are often much more genuine than what exists on the open web's link graph.

If you're in the field of SEO, I think this means social media marketing is a no brainer. And if people aren't recommending and endorsing your site editorially in their Twitter feeds, Facebook updates, LinkedIn groups, answers on Q+A sites, and when socially bookmarking, tagging and voting, I'd be thinking hard about how to change that.

p.s. I still think the social graph overall is still a very small portion of the engines' ranking algorithms, but I think Bing & Google are both racing towards innovation on this front as fast as they can. SEOs should, IMO, follow suit.