How to properly cite a comment from reddit
I believe that your best guess is pretty close to the right answer. According to the APA Style (6th ed.), you should list as much information as possible for non-periodical publications, which you have done well. I think that your resource falls under category "Nonperiodical Web Document or Report", as described on this page of the Purdue OWL's APA Formatting and Style Guide.

However, on the second thought, it seems that a more correct option to use would be APA's electronic sources guidelines for "Online Forum or Discussion Board Posting". Not only Reddit better fits this category, but it also allows you to specify the author of the quote you are citing. Therefore, the optimal citation in question, in my opinion, should be as follows (note that I took liberty to remove date of retrieval as the link you provide is a permalink and, thus, pretty stable):


  Snowden, E. (2015). Just days left to kill mass surveillance under
  Section 215 of the Patriot Act. We are Edward Snowden and the ACLU's
  Jameel Jafer. AUA. Retrieved from
  https://www.reddit.com/r/IAmA/comments/36ru89/just_days_left_to_kill_mass_surveillance_under/crglgh2