Email properties accounted for 49 percent of online ad impressions in May, slightly more than April's 48.7 percent, according to Nielsen/NetRatings AdRelevance, reports MediaPost. Yahoo Mail's share of May's total ad impressions was a whopping 38 percent, followed by MSN Hotmail at 8.6 percent. General community sites accounted for 16.3 percent of online ad impressions in May, compared with 16.1 percent last month; MySpace alone accounted for 14.6 percent of all online ad impressions for the month.

Portals and search engines accounted for 8.3 percent of May's online ad impressions, nearly unchanged from April.

Financial services companies remain the most prolific online advertisers, accounting for 28 percent of ads in May, slightly less than April's 30 percent. Retail goods and services, telecommunications, and web media advertisers each accounted for 16 percent of online ads in May.