During May, the last full month of the U.S. presidential primary season, traffic going to Sen. Barack Obama's campaign Web site outpaced visits to Sen. John McCain's official site by a 4:1 margin, according to data released Monday by market research firm Nielsen Online.

And last month, Obama (D-Ill.) generated more "buzz volume" in blogs and online forums than McCain (R-Ariz.) did, Nielsen said (download PDF).

Obama's Web site had 2.3 million unique visitors in May, compared to 563,000 for McCain, Nielsen reported. The May numbers are the latest data released by The Nielsen Co. subsidiary on traffic to the Web sites of the two presumptive presidential nominees.

Nielsen added that in June, Obama scored mentions in 0.75% of blog-based discussions among Internet users -- nearly double the 0.39% level for McCain. The margin was even wider on message boards and forums: The research firm said Obama was mentioned in 0.89% of discussions there, with McCain registering mentions in 0.34% of those discussions.

"Obama does tend to have a younger demographic that is attracted to him, so it stands to reason that that population would be more active online [than McCain's supporters are]," said Jon Gibs, vice president of media analytics at Nielsen Online. And because of Obama's Web 2.0 approach during the primary season, "this is the first campaign we have seen where the Internet has moved into the middle of the campaign itself," Gibs said.

As a result, Nielsen's usual focus on online advertising spending as the primary measurement of Web-based campaigning by candidates is being usurped by measurements tracking online interactions with potential voters -- for example, the number of people visiting or adding user-generated content to a campaign Web site, and the number linking to a campaign site from Facebook or another social network. "This is the first time where ad spending may be on the back burner," Gibs noted.

But Nielsen is still measuring ad spending by the two campaigns. According to the data released this week, Obama's campaign spent heavily on image-based ad impressions during May, when he was still battling Democratic rival Sen. Hillary Clinton for primary votes. According to Nielsen, Obama placed more than 105 million ad impressions on Web sites that month, vs. 8.5 million for McCain, who had already clinched the Republication nomination.

On the other hand, McCain -- who himself has become more active on Web 2.0 sites -- far outpaced Obama in the use of sponsored search-link ads in May. His campaign posted 5 million of those impressions, compared to 1 million for Obama's, Nielsen said.

The research firm said it also found that 89% of active Web users above the age of 18 are registered to vote, and that they're almost evenly divided between the two main political parties.

Nielsen, which surveyed 36,000 registered voters who use the Web, said 36% reported that they're Republicans, while 35% said that they're Democrats. Another 17% said that they're registered as Independents, while the remaining respondents are either registered as members of other parties or didn't disclose their party affiliations.