Nielsen tracked the entire online display ad market, including the three major gauges of display ad levels: the number of advertisers; the number of individual campaigns they ran; and the number of ad creatives used -- which all increased by 21% over the past year.
In comparison, looking back three years to the first four months of 2006, the number of display advertisers online increased by 43%, the number of campaigns by 49% and the number of creatives by 48%.
Over 5,900 advertisers on average were running online display ad campaigns a month during the first four months of 2009.
Each month saw almost 11,000 campaigns on average being run and 32,000 different ad creatives being used.
The retail and finance sectors, driven by big name high-street advertisers such as Marks & Spencer, Tesco, HSBC and Barclays, are responsible for the lion's share of advertisers, accounting for 24% of all online display advertisers during the period.
Alex Burmaster, communications director of at Nielsen Online, said: "While other media have suffered in the amount of advertising they attract, online continues to thrive.
"Although advertisers are probably getting more bang for their buck through falling online advertising rates, the strong increase in the level of display advertising activity is reason enough for online publishers and media owners to be optimistic about the year ahead -- particularly if the retail and finance sectors continue their heavy activity."