The most important point to remember in citing a comment, whether it is on an article on a newspaper's website, a blog post, a tweet, or any other, is that the original item should be cited as a container. 

As an example, here is how to cite this comment on a New York Times article, "The Real College Scandal": "When we're willing to fund community colleges and their students at the same levels we fund other higher education institutions, we'll start solving the community college completion problem."

APA 

Use Re: to indicate the container post or article, and add [Container type comment] in brackets to identify the format. Use the URL of the comment if possible.

elsiejay (2019, April 26). Re: The Real College Scandal [Article comment]. Retrieved from https://nyti.ms/2LaRz5I#permid=100190506

Additional example:

David, L. (2010, October 29). Re: E-ZPass is a life-saver (literally) [Blog comment]. Retrieved from http://freakonomics.com/2010/10/29/e-zpass-is-a-life-saver-literally/#comment-109178

Chicago

1. elsiejay, April 26, 2019, comment on Richard D. Kahlenberg, "The Real College Scandal," The New York Times, April 25, 2019, https://nyti.ms/2IFoNbv.

MLA

Give the username as the author, and use the phrase Comment on before the title. Add the time of the comment, if available. Note that the URL is that of the article, while the date/time is that of the comment.

elsiejay. Comment on "The Real College Scandal." The New York Times, 26 April 2019, https://nyti.ms/2IFoNbv.