Reduce your bandwidth usage

Hi all

I'm trying to reduce the bandwidth usage of my Android application.
Polling the server application is using the Facebook API on a regular interval graph.

First, I'm already all batched requests into a single request as described http://developers.facebook.com/docs/reference/api/batch/here:

Then I noticed that the response contains documentations HTTP header ETag for each request on a page.
I'm trying to use it, but although the ETag values have not changed (request tags match tags response), the response HTTP 200 code still contains the full content for each page instead of the HTTP 304: no change.

I also tried to caching the value modified but was surprised to see that is not supported.

Any hints on ways to reduce bandwidth usage?
Congratulations
Roy.