The friends system doesn't work as well as we would like so we will be taking it down in a few weeks when we launch the new version of the website. We will then rework it and make it much better, as a part of a much improved social networking set up.
In the meantime, if you want to friend anyone you know, either ask them what username they have used or look up a book you know they own and check the list of Other Bookshelves for usernames you may recognize.