When I signed up for Twitter, it gave me the option to import both my address book and Livejournal friends in order to invite them to use the service. I hate intrusive methods of friend discovery and see this option offered often. Personally, I don't like having my e-mail address(es) shared by friends to random services, so I don't do it to others. However, I'd gladly share my address book if the service used the data (transiently) for discovery instead of for active solicitation. I want to upload my data and see a list of everyone in my address book who has an account with the service, with an option to be immediately linked to them. Instead, we are often forced to search exhaustively for our friends, one by one. I'm sick of it.