Netflix has a feature called Profiles that allows multiple people to use the same Netflix account while keeping their queue and recommendations separate.
Recently, they attempted ( ) to remove the feature because "too many members found the feature difficult to understand and cumbersome, having to consistently log in and out of the website."
It's an interesting example how difficult it is to allow people to maintain multiple personas on a website. At Amazon, the second biggest complaint about the recommendations, next to recommending items already bought from a different store, was that sharing an account or buying a gift for someone that was not marked as a gift would end up mixing up recommendations for multiple people together.
But, the problem is that there is no easy and convenient way to provide multiple personas. Power users might be able to login and logout of different accounts, but most people don't understand or want to bother with that.
Hard problem. It's not clear to me what the solution might be.