In the mean time, another 24 hours have passed in the search for the best creative writing website! (Also in that 24 hours I ate some very unimpressive chicken wings and watched several more episodes of season 2 of Battlestar Galactica, in case you were concerned I was letting my life go to waste ;D )
Figment.com - someone mentioned this one in comments yesterday and all I can say really is "yep, looks like a site" ... to log in though it wanted me to either log in with twitter or my facebook account. I was willing to do so with either... until I had to agree that the related app could post on my behalf. I have a longstanding obstinate policy that nothing can post on my behalf if I can help it, so I was unable to get into figment. The end.
Scribophile - well I had about 3 responses to my introductory post in the first hour, which is more than I've gotten from the other two sites combined since I began this little experiment. Now there are 16 responses. There were also several comments on my wall ("scratchpad?") almost immediately, welcoming me. Friendly folks.
The website itself is clean and professional looking, easiest to navigate so far as well. You have to critique 3-5 other pieces of writing per one of yours you put up (depending on how long your critique is, you get a variable number of "karma points," and it costs 5 to post a story), and as far as I can tell everything gets put in line to on the "spotlight" and stays there till it gets 3 or so "long critiques." -- so I believe one is essentially guaranteed ones writing will eventually move along the conveyor belt into the spotlight and receive at least three reviews. And on top of that everyone's first piece goes in a separate "newbies" spotlight almost immediately.
So going on the "24 hours later" benchmark I used for the other two sites, after 24 hours on scribophile: I've left six critiques, received two critiques (one of which was quite extensive with line-by-line recommendations!) and one short comment on my first story, posted a second story, accrued 18 "reputation points" and have a current karma balance of 1.61.
That 6 to 2.5 ratio is pretty damn good, methinks. I was about to try to calculate the ratio for the other websites but then realized on writing.com none of the critiques I received said anything substansive, and on webooks I've received zero of any kind.
In conclusion, I think I can safely declare scribophile the winner here. If it has a downside I think it's that because of the limitations on what you can post (you can only post two things at once with free membership!) you can really only hope to post your best work / what you are most interested in developing on there. I might still muck around on writing.com for spurious adventures such as 24 hour writing contents (if they'd just write prompts that don't suck -- yesterday it was "a story that begins with 'Listen to the man, with the Golden Voice'..." (and I had my response suspended and censored for pointing out that the phrase the prompt asked us to use had a comma placement error in it), and today it is "write about a statue's revenge"). Fortunately I presumably soon won't have time to contemplate a-prompt-a-day kind of mischief.
Writing.com - Another day or two later and I now have 20,984 pieces of gratuitous pork ("GP"), and the first piece of writing I put up there has been sponsored by someone for potential inclusion in their anthology. But as even those included would have to pay for a copy of it, I believe this falls firmly under the category of "vanity publishing."
Webooks.com - Someone finally responded to my forum post asking if anyone was alive, but by and large the place still looks like a ghost town. Still only ever got one view on anything. This place is dead.