It takes more effort to comment. It "unnecessarily" increases database size. I mean, there is something to be said for "meaningful" comments, but then what is a meaningful comment? What are my comments worth...anything?

Also, one person could artificially add lots of comments. Perhaps you could count the number of users that comment. All of the platforms give you a nice little count of likes and shares, but not comments. I can tell you though that as of now I've gotten comments on all of the different open platforms, but none from any of the proprietary platforms.