I found that I learn a lot by reading forums. I learned about hockey equipment at ModSquadHockey and about musical theater at Musicals dot Net. This is a great way to learn about relatively obscure topics. Unfortunately, this method is also very time consuming because forums are almost completely unorganized. Even if you search for keywords, you have to filter through a lot of noise.
Wikis are much more structured and easier to read. Unfortunately, they require a lot of work because the contributors need to write decent articles and there are the issues of who can edit the wiki and multiple authors of the same article. These difficulties are the reasons you see a lot of incomplete wikis on the web.
A nice compromise between forums and wikis are question-and-answer websites like Quora and Stack Exchange. Instead of having threads like on forums, question-and-answer websites organize by question. For each question, other users post answers. This seems pretty much the same as forums so far. The key difference between forums and question-and-answer sites is that the users can dynamically tag questions and vote on how much they like a particular question or answer. The users do the filtering and that makes the website so much easier and enjoyable to read.
Quora is a very general Q&A site, with the philosophy that you have to stand by your answers. You're supposed to use your real name when you answer and you are asked to give qualifications as to why you are an "expert" in the topic. The site is closed to the public. You have to register and login. I think the reason is it is semi-private is to prevent Quora from becoming like Yahoo Answers which attracts a lot of random, "bad" answers. Stack Exchange is a group of Q&A websites on mostly technical topics. You don't have to login to see the answers, which is why I think Stack Exchange is more popular. I see Stack Exchange websites show up in Google searches whereas I never see anything from Quora. The most famous Stack Exchange site is Stack Overflow, which is focused on non-expert programming (though "expert" is subjective). I have fun reading through the top voted Stack Overflow questions for particular topics like Python and Mathematica. So far I like Stack Overflow better than Quora. Jon Evans wrote a comparison of the two sites in TechCrunch.