I need someone to identify the top 10 online diet/weight
loss/nutrition communities as measured by Alexa ranking and present,
in addition to that list, a summary of all the various features found
that help those communities to work -- i.e., what mechanisms do they
employ to encourage social interaction.

Hi! Thanks for the question.

Here is the top 10 list of  weight loss community websites from Alexa.
The features listed are ones that makes it attractive for members to
be active in the community.

1. Weight Watchers
Community Features: 
- Categorized Boards (ex. age, pounds to lose, )
- Communities are encouraged to swap recipes
- Weekly Meetings for community members
- Tips Exchange
- Motivational Section

http://www.weightwatchers.com/community/mbd/index.aspx 


2. Diet Watch Community (same as Cyber Diet at list #9)
- Ability to Find People with Similar Interests
- Chat Rooms
- Secured message System to other Community Members
- Success Stories

http://www.dietwatch.com/dietwatch/enrollment/index.asp?


3. Health Discovery
- Live Chat Room
- Weekly Meetings
- Chat Calendar for Meetings
- Categorized Topics
- Crisis Topics for those who needs advice or prayer requests
- Recipes Exchange

http://www.healthdiscovery.net/forums/ 


4. Weight Loss Buddy
- Emergency Topics
- Join Specific Teams
- Exchange Ideas with Team Leaders
- Chat with Experts
- Weekly Poll Topics

http://www.weightlossbuddy.com/ 


5. South beach Diet Plan
- South Beach Discussion per Program Phase
- Experiences and Results
- Recipes Exchange
- Medical Issues
- Exercise Related Topics
- Section for Diet Journals

http://www.southbeach-diet-plan.com/forum/ 


6. Nutrio
- Expert Advice and Monitored Boards
- Recipes Exchange
- Exercise Tips
- Fun Challenges

http://www.nutrio.com/content 


7. Weight Watchers Australia
- Success Stories
- Find Friends for Program Support
- Recipes Exchange
- Exercise Tips

http://www.weightwatchers.com.au/WWI_Controller.aspx?Page=1024062 


8. Shape Up America
- Discuss Emotional Hunger
- Maintenance Basics
- Food Activity Log
- Members May Email Experts About Nutrition Issues

Information Source: http://fdncenter.org/pnd/ontheweb/otw.jhtml?id=112300057 

Website: http://www.shapeup.org/members.html 


9.  Featherish
- Diet and Health Plans Discussions
- Discussions on Diet Challenges
- Exercise Tips
- Success Stories

http://www.featherish.com/ 


10. Tops Club Inc.
- Diet Success Story Contest
- TOPS Chapter Meetings
- Chat Room
- Send E-Cards
- Categorized Sections Per Audience (ex. Teens, Men)
- Exercise Tips
- Fundraiser Ideas
- Retreats
- Recipes Exchange
- Rallies
- Recognition Days

http://www.tops.org/desktops.asp 


Alexa Source: http://www.alexa.com/browse/general/?&CategoryID=58486&mode=general&Start=1&SortBy=Popularity


Search strategy used:
Searched at Alexa


I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if you have a question or if
you would need further information.
                                                          
                              
Regards,                              
Easterangel-ga                              
Google Answers Researcher