All things written which include non-trivial expressive content are copyright of their original writers, absent some other factors not present here. You have a

All things written which include non-trivial expressive content are copyright of their original writers, absent some other factors not present here. You have a legal right to quote judiciously for the purpose of commentary. Many individuals on the Internet consider it good netiquette to provide a link to the source of quotes, both for attribution and to allow your readers to examine the quote in context. The general practice of news media outlets when citing HN threads (this happens!) is "A user named $USERNAME commented on [Hacker News|an Internet forum|an Internet forum for hackers] that ..." I have yet to see one provide a link.