Indeed privacy concerns aside, but I completely agree. This will have a huge impact on the web, how can you pass up on 400M users (will probably be 1B in less than two years) and their data on your site? I love the idea of heading to amazon and get a list of movies I haven't seen that my friends which i share movie taste with have rated highly on IMDb. I mean, woha!

Regarding the privacy concern its merely a shift in what is private and what is not - this I think can not be stopped by now.