Historically, there hasn't been much quantitative data available about online events. Until recently when BrightTALK started releasing engagement data sourced across their network of online events.
One data point was the importance of live audience.engagement. Average viewing time in 2010 was 31.3 minutes live vs. 14.5 minutes on demand.
There are 25+ additional data points available from the 2010 online event research here: http://www.brighttalk.com/r/HbS