According to the 2010 overall figures for Digital Ad Spend, courtesy of Darren Herman, of the circa $64 billion Worldwide 2010 Digital Advertising Revenues, Google is responsible for nearly half, i.e. $29 billion or circa 45.7%.
The revenues of the 5 biggest; i.e. Google + Yahoo + Microsoft + Facebook + AOL is equivalent to nearly 64% of the total market share of Online Advertising or two-thirds.
Most of these big players of course own a number of digital properties:
Google = Google + Blogger + Doubleclick + Picasa + Plaxo + YouTube etc.
Yahoo = Yahoo + Delicious + Flickr + Geocities + Upcoming etc.
Microsoft = Microsoft + Bing + MSN + Skype + XBox Live etc.
AOL = AOL + Bebo + CNN + Engadget + Joystiq + Truveo etc.
The breakdown in overall figures is as follows:
2010 Total Digital Revenues