I think the reason is that there is no big problem being solved. Collaborative product recommendation works — a la Amazon, netflix and other sites (and Aggregate Knowledge is also doing great things there).

Socializing is by definition collaborative. Thus, “social networking” takes the offline behavior and amplifies it.

Is shopping social? Maybe some aspects but at its functional level I dont think so because it involves spending money, an inherently personal act.

Which is not to say there is no value, just not enough to see explosive growth.