The StackExchange owners don't want to publicly share too much of the details of their stats... Moderators do have access to some stats that normal users don't, though. I checked and it's okay for us to share a little of that data, especially when "the data is helpful to the users for guiding activity" (quote from Robert Cartaino in a chat).
I don't have stats on where registered users came from, I do have some fairly basic stats on where visits originated from. About half are from SO or elsewhere in the SE network.
So, if, say, you posted something about bicycles.SE on your blog and wanted to find out if it looks like it generated very many visits in the week following the post, we should be able get that for you...
Additionally, they've recently started putting up some stats on Area 51 that might interest you: http://area51.stackexchange.com/proposals/2305/bicycles