To the original question: I think it just comes & goes over time. There'll be periods with lots of reviews and periods with few. There'll be periods with lots of women at any agency and lots of indies advertising, and periods with hardly anyone.
I'm sure what some have said is very true - of ALL the folks seeing any SW, how many are even aware that boards exist? If they do, there's probably a multitude of reasons to not write a review - some may feel shame, some fear exposing themselves, some fear leaving a digital trace, some simply may not be bothered (I for one don't review every restaurant, shop, or service I receive; I've literally reviewed ZERO services outside of this and other forums, lol!).
To the folks complaining: personally, I think Perb has cleaned up quite a bit and for the better. It'll never be perfect and there are MANY SW's that I've met and I respect that HATE the review culture - demeaning comments, insinuations over drug use, complaints over minor details, etc. And maybe some clients who read this simply don't want to be part of it too. Personally, I wish the forum would allow more SW to respond to negative reviews - I've seen a few where the woman bluntly says how the guy acted and why that made them uncomfortable. Sure, that risks spiraling into a he-said/she-said, but it gives folks a fair chance. Other review spaces in the world allow that. (this forum does allow paying SW's to respond, I just wish it would allow more from non-paying too IMO).
For a long time, I just stayed off the board, it wasn't something I wanted to be a part of. Then I saw some improvements (still not good enough IMO) and thought I'd add a little bit. Even if my date went poorly, I will usually not write something fully negative BECAUSE how someone acts can very often be affected by how I act. Was I inadvertently weird? (I'm an awkward duck at the best of times, lol!) Did I say/do something that triggered her to remember something else unpleasant? We're all humans, not machines, so even on dates that were so-so, I will give the benefit of the doubt that someone else could very well have a great time with someone even if my date wasn't. I also know that negative reviews can affect others' response WAY more than positive ones (I feel it, whether its reviews here or on Y3lp or Go0gle). So unless it was truly, irreparably horrid, I simply won't do that because of the reasons above. Again, if you don't like that, too bad - feel free to ignore ALL my reviews then

If folks don't like how I write review, TFB. Go see her and write the review how you want to.