A great way to grow social networks is virally, with each new member who joins inviting a bunch of contacts to come and join them there. It means each new member fought for could mean five members.
We incorporated address book fetch functions on our site to make it easy for members to invite all their contacts. It was fairly easy to do and we allow contact lists/address books to be fetched from Hotmail, Google Mail, Yahoo Mail, Outlook and more. It is fairly easy to also add the same to fetch contacts from Facebook and Plaxo and more.
If you want to see how it works on our site go to
https://www.wecando.biz/my_contacts.php?page=2 (registration is required to see it, but this takes just a few minutes).
I'd be happy to share how we did it up PM.
IH