Is it recommended to build a forum using node.js?
I've heard that node.js is really fast for applications like chats and real time apps. I was wondering if node is a good choice to develop a forum or a q&a site like StackOverflow?
Yes it's perfectly possible, you don't have to implement your own web server like somebody said, you can just create it using something like Express (as a framework) and Mongoose (as a MongoDB ORM) or whatever you choose.
I think Node.js is a good solution for such a case because of its high concurrency (and Stack Overflow is a site where thousands of users are online in the same time probably). So yes you can achieve this with PHP, Ruby and Python etc, but I believe Node.js is a better fit (that's my opinion, based on the success stories I've heard which were based on Node).
Node isn't just a solution for building small services, here are some good examples ( http://blog.Nodejs.org/2011/10/04/an-easy-way-to-build-scalable-network-programs/ ).