According to a Goldman Sachs survey, social sites have very little influence over online shopping decisions. Search engines, and recommendation engines have a much greater influence.
As long as Google has influence over shopping habits, it will continue to pull in big ad dollars. Advertisers tend to want to advertise where there’s a chance a purchasing decision can be influenced.
However, in the not too distant future Facebook could become an e-commerce juggernaut with more influence over online shopping. If and when that happens, Google should be really worried.