When someone uses Bing's search engine to look for a new car or a book, she can see which ones her friends liked.
Wait, what? This is a good idea how exactly, apart from Facebook and Bing cashing in big on gullible marketers who still think that "personnalized adverts" are the next big thing?
Newsflash: being friends on the web doesn't say much about what people have in common. Parsing my friends' list on Google, I can't honestly find more than 10 people that may suggest things that I may like. They also tend to be my real-life friends, and will give me this information in REAL LIFE CONVERSATIONS.
Pitching specific search results based on user profiling is also completely broken (in my case). I happen to work on two very different topics, and my Google searches for either topic are frequently polluted as Google thinks I am looking for information related to the other one. Which forces me to reset my Google search preferences and cookies every now and then. Also, Google will tend to rank up contents similar to websites/articles I've visited. When researching political topics (which is my job) it gets in the way of getting a clear, comprehensive picture of the issue at hand.
Even for more mundane stuff such as online shopping, "personalized" ads are usually pointless. Case in point: last week I made an online reservation for a hotel in a certain country. Now half the ads I see are for hotels in the same city. Great, except that I've completed my trip and won't be going back there for the next 2-3 years...