What is a better model for using tags on social content websites?

Tag Cloud? - Since this caters to the long tail of users looking for 'interesting' content? eg: flickr

Category tree? - Since this gives more targeted/specific information to users 'looking' for specific information. The categories again could be designed through user generated tags. eg: subreddits, digg

Combination? - Is the 'best of both' a good model?

Or, will a tag-search based model suffice?

Or, does it depend on how domain specific the social content platform is?

Personally, I don't use tag clouds. I have browsed many sites with tag clouds and I view them as mere space fillers.

I think the tag cloud is self explanatory in the sense, the font of the individual tags depends on the amount of content under a particular tag. It's a visual representation of the category system where the amount of content in each is represented by a number in brackets in front of each category. Both these techniques have their own pros and cons as one is more visual while the other asks the user to find out where he wants to go. I think a combination of both might be the optimum solution, where the content is provided under tags which are pretty broad and category and sub category takes the user deeper and is more useful for users who are looking for specific information.

tags and categories are two sides of the same ontology coin

with tags, the ontology is built from the bottom-up without a hierarchy or direction

with categories, the ontology is built from the top-down and usually a hierarchy

at the end of the day, they are both simply a subset of the english language associated with content