There is a great article on CNNMoney.com about Wikia which was formed by the people behind Wikipedia, Jimmy Wales and Angela Beesley. Wikipedia is a non-profit service and you won’t even find Google AdSense on the site. It relies largely on donations to keep itself going.
Wikia is a hosted wiki service based on MediaWiki software (the same engine that powers Wikipedia). Users can create wikis of their own on Wikia and have them hosted there for free. There is a review process to ensure that there are no similar wikis already in place (if there are you will be encouraged to join those wikis instead). The revenue model seems to be advertising revenue (Google AdSense is on the site) using the huge numbers of people who visit the many wikis on the site each day. There are a number of really popular wikis on Wikia including the Muppet wiki.
What strikes me about wikis is that there is a strong sense that the wiki doesn’t belong to one person but rather to the community that updates it. With a blog there is a clear sense than the bloggers control the blog and pretty much own the process but with wikis it is the opposite. A person may establish the wiki and contribute to it but it belongs to the community at large.
(Source: Social Media)