Have you thought of adding an "Invite Friends or Family" button or link? I wanted to tell my parents and brothers and sisters about Fuelly, and searched every page of the site, but found no way to do this. I ended up just sending them an email with a link to the site and my username so that they could find my stats and add me when and if they sign up. This would make doing this a lot easier and drive membership in the site. Thanks.
We get this suggestion a lot but we've been very hesitant to add it just based on our personal experience with social sites. The social flood has slowed down now, but we used to get automated invitations to social networks seemingly by the dozens every day—to the point that social spam was rivaling pitches for ******.
I appreciate the approach that you took, and I pay much closer attention to a personal email from a friend or family member than I do from an automated mass email from a website. That's why we added this FAQ:
I realize it does make it a bit harder to add someone as a friend on the site, and that is a downside to our approach. But we're trying to keep everyone's email inbox as useful as possible by specifically not adding an invite everyone I know feature. I can't say we won't ever add it, but that's our thinking up to this point.