If you check the response headers for this request, Github provides links to pages under the Links heading.
Link: <https://api.github.com/users?since=135>; rel="next", <https://api.github.com/users{?since}>; rel="first"
I believe that with their api v3 Github moved to a hypermedia api.
Github Hypermedia API
EDIT
This is beyond the scope of this question, but is related to it. To learn more about the hypermedia API and REST. Take a look at these Steve Klabnik slides
http://steveklabnik.github.com/hypermedia-presentation/#1
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