The GitHub API documentation says that the url
https://api.github.com/users
will give all users in the order they signed up, but I only seem to get the first 135
If you check the response headers for that request Github provides pagination links under the header Links
Link: ; rel="next", ; rel="first"
I believe since their api v3
Github has been moving towards a hypermedia api.
Github Hypermedia API
EDIT
This is beyond the scope of this question but its related. To learn more about hypermedia API and REST. Take a look at these slides by Steve Klabnik
http://steveklabnik.github.com/hypermedia-presentation/#1