I'm sure there are stats...somewhere.
What I see is an email notice with every new registration. They average out to about a dozen a day. And those are the ones who make it thru the spam blocks. There are about 20 a day that are blocked from registering by spam blocks we have in place.
Of these dozen or so a day, not all are 'real' members either. Some are spammers as well, some are just 'guests' who want to view the forum and most never activate their accounts. So many come to see and read, but never post. I was like that for many months before I ever got the nerve up to post.
So it's really hard to tell who are 'legitimate' new members.