The Pew Internet & American Life Project just published a study of online engagement in the response to Hurricane Rita. There were two main points:

1) 13 million people gave money online in response to Hurricanes Katrina and Rita. The number of donors is up 53% overall since 2005.
2) 7 million people (5%) set up their own hurricane relief efforts using the Internet.

Here are the details.

http://www.pewinternet.org/pdfs/PIP_Katrina.DateMemo.pdf