This is the classic web community. It's revenue streams are the same as those found in the Standard Web Publisher model, although with it's more dynamic elements it may be able to make some money on transactions. The variable costs are lower as you have to generate less magnet content, but it has a higher technical/initial cost as you have to build web services to allow users to interact. Depending on your community there may be other revenue streams which are defined by the community's theme. In this sense revenue streams may eventually be "discovered" as the scope and nature of your site evolves. Does this model really work?

• Slashdot.org: One of the older online communities, this website has so many techies reading it a day that a site linked to Slashdot may experience the Slashdot effect. What is the Slashdot effect you may ask? If one day your traffic goes through the roof and a strange burning smell is emitted from your server room you may be experiencing it. Even mighty websites can be taken down by the sudden rush of hundreds of thousands of users requesting pages over the course of an hour. With a lot of media attention in the US turning to online communities slashdot is getting a fair amount of media play. Is it profitable? I honestly don't know. They have adopted an advertising model and a subscription system which does net them some dough. I tend to assume that it is enough, but I doubt that the website is bringing in the millions of The New York Times.
• Photo.net: Founded by Philip Greenspun, the man who created what would eventually become OpenACS, this website is a hub for photo enthusiasts. After a few years of success it eventually become the bully pulpit from which Philip would publicize himself, his company, and his theories. For a website without a major corporation behind it, it gets a huge # of hits a day. For a short bit it was sponsored by arsDigita, but as the end neared Philip spun it off into it's own company. Although Philip has the money to keep it afloat I suspect/hope that the website is somewhere near breaking even.
• Yahoo! Groups: A community site where you can build your own community Yahoo! has continued to build one of the most solid websites on the planet. While most dot-coms tried to publicize themselves and eventually ran out of money Yahoo! has quietly lowered it's profile and gone on to being a solidly profitable company. Although Yahoo! Groups is only one part of the extensive Yahoo! website, this is a classic example of a company who uses it's groups and many other features to build a profitable whole. I Yahoo!, do you?*

* Tristan Cohen is in no way affiliated with Yahoo! corporation