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.
"It's been hard to measure the success of online advertising campaigns," said Adam Spielberger, executive vice-president of interactive marketing at Special Ops Media, a New York interactive advertising agency.
"[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 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 used the data to decide how to target their next round of ads or where bands should tour, YouTube Insight product manager Tracy Chan said.
"YouTube is becoming the world's biggest focus group," she said.
The YouTube data is more specific than that bands typically received from television and radio, Ben Patterson, who worked on Weezer's digital marketing strategy, said.
"What's distinct about YouTube Insight is the immediacy of the information and the discovery element - how viewers found the content," he said.
Weezer, an alternative rock band, was founded in 1992 in Los Angeles.
Its Pork & Beans video is purposely filled with references to the YouTube amateur celebrity culture and appearances by YouTube celebrities.
They include Chris Crocker, famous for his teary plea for the media to leave Britney Spears alone, and the guys who did the Mentos/Diet Coke explosion experiments.
With Insight, Weezer learned that people who watched Pork & Beans were predominantly from two age groups: under 18 and between 35 and 45.
At first, most found the video by going to YouTube or Google, but many linked to it via technology news blogs such as Valleywag.
After three days, 38% of viewers had discovered Pork & Beans not on YouTube but on other websites where people had posted it.
Weezer's experience on YouTube will affect marketing decisions this summer as the band, whose latest CD came out last week, prepares to go on tour 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.
The Insight information, he said, raised questions.
"Should Weezer curtail advertising in traditional spots such as local alternative weeklies and banner ads across music blogs and sites?
"Would money be better spent advertising on technology sites such as Gizmodo?"
Ideally, the data provided by YouTube Insight will "help identify better who true tastemakers are", said Mr Patterson, who is president of DashGo Inc, a Santa Monica, California, company that provides digital distribution and marketing to independent artists.
"What really sparks a viral campaign? What is the match to the flame?"
- Michelle Quinn
