Buy.com has launched a sister site called Yub.com (took me a second to realise that it was just Buy spelled backwards).
Yub is basically a case back loyalty program, with the addition of:
Yub.com is the world’s first shopping social network, which means that when you hang out on Yub, you can interact with other shoppers, make friends with those you trust, and earn rewards together. Buy something a friend has reviewed and you both earn 1%.
This could add some interesting themes on “those who bought also bought”. With a large enough network I could ask “those in my social network who bought, also bought”. I think we will start seeing more applications which use social networks as a platform.