What would entice you to invite your friends to the site?

I am a believer that a community / dating / social networking site is only as good as it's members.

If you have the prettiest site built with the latest technologies but you don't have many members, you do not have much.   So please take a moment to answer this question -

 What would entice you to invite your friends to the site?

I have tried a couple of programs, neither of which was very successful -

• 
  
  Webmaster Affiliate Program - With this program, webmasters would make money if they referred paying members to the site.  Similiar programs have been very successful for other sites.
  
  
  
• 
  
  Member Rewards Program - With this program, members would be rewarded with gift cards just for inviting a friend who signed up for the site.  Many new members are referrals from other members so it was just a way of rewarding people for doing what they were already doing.
  
  
  

Some social networking sites have a feature that allows their members to send invitations to everyone in their address book.  I always considered this to be a bit much, but it must work for them since a lot of people have been adding this to their site.

Please give your opinion on this.