iWeb is not a powerful enough tool to make a site like Fluther. Fluther uses many different coding languages and server-side user information and dynamic content stuff like most Web 2.0 sites, iWeb doesn’t allow you to edit the source code so it will be very difficult to add features like Fluther’s, also you will have issues with server-side coding stuff if you plan to use .Mac.
I suggest you begin by buying a program like Adobe Dreamweaver, also it is best to start with simple, less ambitious projects until you get used to web-development.
Alternatively you can find an online service, like Mirza suggests, which allows you to create a free forum or simple website like Bravehost or Freewebs.