Ask HN: Whats easy and reliable contacts importer/inviter service?

Hi, We need to allow our users to invite friends using their address book from popular email services like gmail, hotmail, yahoo, rediffmail and also from social networks like facebook, orkut etc. There are many sites like Openinviter.com Invitebox.com Cloudsponge.com ContactGrabber.com GetMyContacts.comsuper-tell-a-friend.com inviteallfriends.net smartaddon.com (found them by searching Google) However, I am not sure of their reliability. Im wondering what most successful and serious startups are using to allow their users to invite their friends. It should be reliable and easy and work with PHP/Yii. I would love it if its free but its also ok if there is a small monthly fee. Any suggestions?