Here is an excellent ppt to present best practices for designing social networks.
A good complement to this presentation is the following list of Social Network Design Guidelines, published by Cameron Chapman from Smashing Magazine on July 13th, 2009. 
1. Engage Quickly
What’s It For?
Give Visitors Something To Do
Promote Interesting Content From Friends
Make It Easy To Find Friends
2. Let Users Express Themselves
Profile Pages Should Promote Personal Expression
Promoting Individuality In Applications
3. Be Dynamic
Have Regularly-Changing Content
Update Content in Real-Time
4. Allow Friends To Be Grouped
Let Users Define Groups
Create Automatic Groups
5. Use OpenSocial
Provide More Applications To Users
Let Users Take Their Profiles Anywhere
6. Make It Easy To Communicate
Provide Multiple Means Of Communication
Foster Conversations
7. Show Only Relevant Information
What Really Needs To Be Here?
Don’t Overwhelm Your Users
Give Users The Ability To Filter
8. Make It Easy To Take Action
Emphasize The Desired Response
Make It Easy To Find Things
9. Show Avatar Photos
People Like Seeing Other People
10. Include Ways For Members To Connect
Include User Groups
Provide A Member Directory
Further Resources
Social design best practices
Applications of usability principles on a social network
Article twitted by Dr Yuping Liu, Marketing Professor at Old Dominion University, Norfolk VA (to check a list of her publications)
