A long time complaint many users (and people who refuse to become users) have had with Facebook, is that there is no one type of "friend." The fundamental problem of facebook was also the premise of Facebook.
"Th basic issue is this," explains Zuckerberg, "One of the greatt things that Facebook has anabled people to do is connect with all their friends and stay connected with all of them all at once, but in reality people's social worlds don't look like one big thing."