1. users must be able to determine the social object within the first 10 seconds (what is your social object?)
2. it should be clear what you do here around the object, eg Buy and Sell on ebay (what are the verbs?)
3. social objects must be shareable, eg permalinks or widgets/embedding (how do you share?)
4. service should support virality – turn each invitation to join into a gift, eg when you are sending your contacts a funny YouTube video you are sending them a smile
5. charge the publisher not the spectator – nobody wants to pay before they see what they are getting
More on what makes a good social object.