Getting all GitHub users through github-api

Both of the existing answers are 100% correct, but I would advise you to use a wrapper for whatever language you happen to be doing this in. There are plenty of them and there is an official one for ruby (Octokit). Here is a list of all of them.

If you check the response headers for that request Github provides pagination links under the header Links

Link: <https://api.github.com/users?since=135>; rel="next", <https://api.github.com/users{?since}>; 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

Please use since parameter in your GET request.

https://api.github.com/users?since=XXX

Probably it's done this way to limit the resources needed to handle such request. Without such limit it's just asking for DoS attack.