In order for a compilation to be protected by copyright, it has to have a modicum of creativity. A device that just automatically copies and displays material thrown at it does not add the creative spark that is necessary for protection. It's possible but quite unlikely that a highly-moderated website has human intervention that structures the posts in a sensible manner.
There is a very remote chance that you had to assign copyright in your contributions to the website, in exchange for them posting the material, but otherwise you retain the copyright in your own contributions (so of course you can make it into a book). However, you would need permission from the copyright holder to publish their material. If, for example, you have an extended discussion with User A, you could not publish User A's contribution without permission. You could publish your responses, which might include brief extracts of the original questions (this may be legal under a fair use analysis, depending on the "heft" of the question that you copy into your answer). For instance, if there is a series of short questions like "What are the wages of sin?", you might be able to copy the stimulus questions that you are responding to (and probably did so in your answer, so that readers could figure out what question you were answering).