A lot of sites these days will ask users to invite their friends. Twitter, for example, even dedicate part of their right sidebar to constantly ask me if I want to connect my Gmail address book. LinkedIn spent a lot of real estate in an attempt to get me to go through my address book for potential new LinkedIn connections.
Don't ask me to invite my friends, please. If I want to, I've probably already done it. Especially if I've been using the service for several years.