Specifically, Yahoo Mail accounted for the single largest proportion of May's ad impressions--38 percent--while MSN Hotmail garnered 8.6 percent and Juno Email on the Web drew 1.1 percent.
General community sites captured 16.3 percent of the impressions tracked last month by AdRelevance, compared to 16.1 percent last month. MySpace was the clear leader in this category, claiming 14.6 percent of all online ads served in May. Portals and search engines additionally accounted for 8.3 percent of May's online impressions, versus 8.4 percent in April.
Which companies advertise online? Financial services companies continued to be heavy online marketers, accounting for 28 percent of the online ads tracked by Nielsen//NetRatings, marking a slight drop from April's 30 percent share.
Advertisers in three categories--retail goods and services, telecommunications, and Web media--each accounted for 16 percent of online ads last month. Retail and Web media ads increased slightly from last month--15 percent and 13 percent, respectively; telecoms spending dipped slightly from last month's 17 percent.
The overall number of display ad impressions tracked by Nielsen//NetRatings AdRelevance rose to 197.7 billion last month--nearly flat from April's 197 billion, but more than double the 93.1 billion in May of 2005. But Nielsen//NetRatings cautioned that the surge in number of online impressions it measures doesn't necessarily reflect a corresponding growth in online advertising. Rather, the company stated that changes in its methodology might account for the apparent ballooning numbers. For instance, Nielsen//Netatings AdRelevance now examines more sites than it did last year.
The AdRelevance data doesn't include ads served on proprietary AOL pages, but counts ads that appear on AOL pages accessed via the Web.