I guess it was 1999 when this was written, though, so the norms for online reviews had not yet been established.
It's one thing to fake a review, but there's nothing wrong with using that spot to talk to your readers.
I'd argue they carried the same social role as blogs. To me, the key vision is that online, self-published essays could be widely read and influential.
-edit- Actually referencing the book, they do eventually become part of "newsnets", being paid for their work and gaining a wider audience.
It also describes them using temporary anonymous accounts, injecting deliberately inflammatory comments into discussions to provoke a response. >_>
On a more salient point, I always find it interesting to compare the idea of anonymity in the Ender world compared to what we have now. I'd be curious in seeing Card's take on it; despite how prescient the idea of a blog was, I don't think he ever conceived of how massively outweighed the signal could be compared to the amount of noise.
It wasn't a 'blog' per se, more of a forum, I think ... I only dropped in once or twice and my recollection was that the discussion was (at the time) more about his religion than about his writing.
But if he'd just wanted to speak to fans he's certainly had an outlet to do so for a long time, and had it at the time the review was written. Presumably he chose to write the response in the form of an Amazon review consciously, and not for want of some other venue.