Three days after Weezer posted its music video “Pork & Beans” on YouTube, 2.2 million people had watched it. And 65% of them were men.

The heavily male demographic surprised the band’s marketing team, which three months ago wouldn’t have been able to find out about the gender, or much else, of the people clicking on one of Weezer’s videos. A feature called YouTube Insight, introduced in March, gives YouTube account holders who have uploaded videos to the site a range of statistics, charts, and maps about their audiences.

The data bring a little science to what has been the art of viral marketing and the potential for big changes in how bands, television shows, movies, and consumer products are promoted on the Internet.

Until recently, “it’s been hard to measure the success of online advertising campaigns,” the executive vice president of interactive marketing at Special Ops Media, a New York interactive advertising agency, Adam Spielberger, said. Now “people are trying to dig down into the numbers.”

Insight is part of a trend on the Web. The social networking giant Facebook offers account holders a weekly report that, like Insight, is free — and has a similar name, Insights. The information it provides is used by individuals and companies that have Facebook pages and want to hone their marketing.

Before Insight, success on Google Inc.’s YouTube was measured primarily in one way: by the sheer number of “views,” or times a video was watched. The data available through Insight include age, gender, and geographic location as well as the identities of the Internet sites that viewers came from and where they went after watching a clip. Marketers and advertisers use the data to decide how to target their next round of ads or where bands should tour, the product manager of YouTube Insight, Tracy Chan, said.

“YouTube is becoming the world’s biggest focus group,” Ms. Chan said.

The YouTube data are more specific than what bands typically can get from television and radio, the president of DashGo Inc., a Santa Monica, Calif.-based company that provides digital distribution and marketing to independent artists, Ben Patterson, said. Mr. Patterson worked on Weezer’s digital marketing strategy.

“What’s distinct about YouTube Insight is the immediacy of the information and the discovery element — how viewers found the content,” he said.

With Insight, Weezer, an alternative rock band, learned that people who watched “Pork & Beans” just after its release May 23 were predominantly from two age groups: under 18 and between 35 and 45.

Mr. Patterson said Weezer wanted to figure out how to target the same demographic groups that watched the video and how to reach out to those that didn’t.

Ideally, the data provided by YouTube Insight will “help identify better who true tastemakers are,” Mr. Patterson said.

“What really sparks a viral campaign? What is the match to the flame?”