I'm here because of stack overflow. I wonder how many of our first time questions come from people who have other accounts on SE?
Here's your info, thanks to Rebecca Chernoff:
1247 (non-employee) users on Gardening 1122 registered users 1030 users have an account on SO or SU or SF 1003 users had SO/SU/SF before Gardening
I didn't ask for users who had accounts on SE 2.0 sites prior to Gardening, but as you can see from the above, that number is going to be pretty tiny.