I have 2 github accounts, one personal that I use very seldom, since I prefer to put my free software on gitorius.ORG in Norway rather than on github.COM in the USA, and another for work, since my boss puts his company's sources on github.
In anycase that means 4M-1 users, for 4M accounts.
I was personally surprised as i thought there are more users.
But still 4M developers are quite a lot. Love using Github.