Netlash published today "5 tips to a successfull Online Community" (Deutch).

Many times I have tried to set up such communities (Bicycle Club, Work's Intranet, MBA distribution list, family, customers sites). very often those communities have died even before they were born.

There is one principle that I have kept out of all those web experiences : people use the communication tools only when they have a problem and when they are looking for a solution.

This is the "What's in in for me" principle : I have only recorded activities when people get an advantage out of the communication:

• Forums : people asking questions see their problems solved, get information. Answering people raise their visibility. That's why on Linked In you can be flagged as an expert. Another example I saw was during MBA exams: the Forum was suddenly alive.
  
• Distribution Lists are all a sudden used when someone wants to sell his car or quickly organise a barbecue.
• Social networks are useful when you want to see what the others are doing (a kind of voyeurisme)
• Viral marketing is like the new way of telling jokes (or stories). Why do people tell stories ?

I guess you can continue the examples by yourself : People communicate when they hope to get something out of it.

If you understand this "What's in it for me" principle, you understand that the web can be a very powerful tool to quickly find a solution to your problem. The range of problems is very wide : finding love, compiling Linux, buying Viagra, finding the perfect vacation, stay informed, find which WII game is cool and which one sucks... Identifying he community of people who have the same kind of problem than you can only bring you closer to the solution. This is for me the true engine of communities.

I use to say to my wife (as a joke ?) "Everything always comes back to 3 letters : SEX".
But in that logic that there are also 3 other letters : "EGO".

And than I guess there are some things like "Right man at the right place" and "Time to Market" also applicable to Online Communities...