It looks like typically 2 or 3 sign up a day, judging from the last few days. However, I don't think you can compare, because that site is for a different genre (less interested in any lengthy debates), and seems to have a lot more young people.
Some don't seem to post anything. I first signed up to this site years ago, with a cool username, and never posted a thing. (Someone else here, uses that username, now). It took me a while to have the courage to post anything.
Perhaps large numbers are signing up because Flodin recently warned about apostate websites. It would be interesting to see if it continues at that rate, because if 20 people are signing up per day, that is 7200 per year. And if only 1 in 10 people who visit the site, sign up, that would indicate potentially 1% of publishers around the planet are checking these sites out, in any given year. I suspect Simon would know more about what is going on, how many "hits" etc.