The San Francisco Business Times reported (August 28, 2006) that$4.5 bb in giving was recorded in 2005. Much of this was disaster related. More than 1/3 of giving for the tsunami and about half of all Katrina-related giving happened online.
Online giving was only about 2% of total giving (More than $260 bb in the USA in 2005). But it grew from $500 mm in 2000 to $4.5 bb in 2005 - not a bad rate of growth.
Another note, Sarah Duxbury, who wrote the article for the Business Times references Network for Good as the data source. Online gift transaction processors, such as NFG, GlobalGiving, Kintera and others are important new data sources for us.