At the risk of being pilloried: Social signals are not the be-all and end-all of the paid or organic SERP. For all the hyper-ventilation on the topic, social signals are but one signal among many to the ultimate treasure map.
Personalization is the ultimate treasure map.
Search engines seek to provide a SERP that best responds to the intent and interests of each individual user. An answer when the user's intent is informational and unambiguous (think: "Madonna's birth date"); a range of potentially valuable options when the user's intent suggests as much (think: "car insurance").
The engines have progressed from serving the same results to everyone, to serving personalized results based on geography, social signals, and past click-through behavior.
But couldn't Google take it a great deal further than that?
Google knows not only whether this person tends to click on Walmart links, but whether they buy from Walmart when they get there. They further know where else this person will shop, and where they never shop.
Does this browser ever transact online? For what kinds of sites?
Imagine what smart advertisers could do if those types of targeting options were available in paid search advertising? My ad copy could vary depending on the browser's customer profile. I might serve different copy and pay different amounts for the privilege, depending on whether they were a loyal customer of mine (brand heavy ad text) or a loyal customer of my competitor (Like my competitor? You'll LOVE me!") or someone with divided loyalty (promotional offers, perhaps?)
For online-only business, the value of browsers known to transact online would be materially higher than the value of those who don't. The value of folks who never transact online might be significantly more valuable to brick-and-mortar businesses with a less crucial online presence. You might pay more to make sure you retain your most valuable customers when they search than you would for someone who is a bargain hunter who returns everything.
Behavioral data has always been FAR more valuable than demographic, psychographic, or contextual data. Imagine the power of combining the in-the-moment intent of search with Google's browser data and your own customer data!
RTB comes to Google and Bing...
Is this a Brave New World? Terrifying? Exciting? Inevitable?
I'm taking bets :-)