Posting something on a website does not make it public domain. Typically, the website's Terms of Service will require that you grant the website operator a license to use any content that you post on the site (so that they can display it to other users). That license does not extend to other visitors of the same website.

Of course, in practice, it's very unlikely that someone would take you to court over copying a website comment. But if someone posts, say, an original work of art or a short story in a comment thread, you should be aware that it is still protected by copyright.