Google Apps administrators have some useful controls and tools to track where documents are being shared. Often there is a fundamental policy about sharing outside the domain (for example, to limit exposure of privileged or private information).
An administrator can set up the Drive app to prevent sharing beyond the domain.
Note that this does not revoke existing shares! If you are closing off a loophole in your security, then you can discover the documents and files that have been shared publicly (beyond the domain) using a script or custom app like the General Audit Tool.
This also provides a means of enabling sharing of documents and files by exception. Keep the domain normally secured so that users cannot share outside the domain but open the gate briefly to share a document or folder publicly and then close it again. This is handy if you want to use Google Drive Hosting on an otherwise closed domain.