Both bebo and LinkedIn have little notes when you invite people to be your "friend" telling you to only invite people that are actually friends or that you actually know. LinkedIn even has a punishment for inviting people that you don't know:

recipients can indicate that they don’t know you. If they do, you’ll be asked to enter an email address with each future invitation.

I prefer the way that livejournal and stumbleupon do this, where friendship is a directed relationship -- you can unilaterally declare that you're someone's friend, which gives you access to content of theirs that was already public, but does not subject them to any of yours. The use of a directed graph seems more elegant, but LinkedIn's punishment scheme is pretty clever.