Thursday, April 24, 2008 02:10 PM
You probably want to start with a Use Case. Each Use Case takes care of ONE function that a User can do (login, edit profile, send message, read message, etc etc).
Then, you would create a Sequence diagram for each Use Case.
You are on the right track and Sequence diagrams are not designed to to take care of ALL the things that can happen in all cases with just one diagram. They start the action the user takes to perform an action, then show the Object communication from object to object necessary to carry out that action.
There are some good tutorials on UML diagrams out there... you can do a Google Search on Use Cases and Sequence Diagram tutorials and get more specifics.
Hope this helps.