Februs:
I don't have the precise stats to hand but I'm sure the overall majority of those who comment on the LSF challenges are authors in their own right rather than readers, except here the comments can be made by anyone rather than a tiny elite predetermined few.
Well, to be fair about it, anyone could volunteer to be a reader/reviewer in the newsgroup's Short Story Contest, as long as he/she was willing to put in the time and effort to read his/her assigned stories and write a comment on each one.
So it wasn't an "elite predetermined few," I was a reader/reviewer for the last couple years of the S.S.C. (as well as a contributing author), yet I wasn't part of any "elite" there that I was aware of.
However, I do prefer the way that writing Challenges are handled here in the Library, since it seems that just about every entry will manage to receive at least 15-20 comments from the readers, while in the S.S.C. there typically would be 3-4 comments on each story from reader/reviewers, plus whatever ones were offered by the membership in general when it was posted in the newsgroup, perhaps 2-3 more with luck.
Of course, there's simply more emphasis on commenting, and even writers responding to comments, here in the Library than on any other spanking story site I've encountered...

--C.K.