Quote from the AGPL preamble:
It requires the operator of a network server to provide the source code of the modified version running there to the users of that server. Therefore, public use of a modified version, on a publicly accessible server, gives the public access to the source code of the modified version.
edit: according to a comment below, Google is already sharing their improvements without being obliged to do so. Nice! You can't always count on goodwill though, so my point stands.