1) there is no real "tragedy of the commons" in the general case. Google Elinor Ostrum to verify. Even Garret Hardin (originator of the concept) has conceded this, at least in part. The crux is that the "commons" as described by Hardin has essentially never existed in the form that he wrote about - things that are "commons"-like are actually always a complex mixture of law, tradition, culture and social sanctions governing their use. When people screw up "the commons", it's not because there are no mechanisms to prevent it, it is because these people have chosen to ignore them, and have made extra effort to do so.
2) You're seriously claiming that Reddit, a privately held company with sysadmins, subreddit moderation and user voting, could be a venue in which what happens is "a tragedy of the commons" (should such a thing actually exist)?
Reddit is privately held but has quite of an impact on the public. Facebook is the same, as are other notable walled garden/social networks. There’s a term for that: utility company.
2) You're seriously claiming that Reddit, a privately held company with sysadmins, subreddit moderation and user voting, could be a venue in which what happens is "a tragedy of the commons" (should such a thing actually exist)?