New York--B-to-b online advertising decreased 17% to 3.2 billion ad impressions in May, compared with a year earlier, according to Nielsen//NetRatings AdRelevance data released Wednesday. In fact, it was also the only category of 13--a list including retail goods and services, financial services, and travel--to decrease. Every other category increased by double-digits. Web media ads had the most modest increase, rising 38% to 12.1 billion. Of the b-to-b advertisers, Oracle Corp had the most impressions (788.9 million).