Be afraid. Be very afraid. Sort of. Harris Interactive suggests YouTube and its ilk are in fact beginning to leech viewership from traditional video outlets.

Over four in 10 (42 percent) online U.S. adults say they have watched a video at YouTube, and 14 percent say they visit the site frequently, according to the research firm. The kicker is that one in three (32 percent) of the frequent YouTube users say they are watching less TV as a result of the time they spend there.

Further compounding the problem for TV and advertising, YouTube usage is greatest among the group already hardest to reach through television advertising: young males. Over three-quarters (76 percent) of 18-to-24-year-old males say they have watched a video at YouTube, and 41 percent visit YouTube frequently.

YouTube still isn’t able to monetize its viewership, however, Harris Interactive said, a caveat that should provide only minimal comfort to TV providers.

YouTube is considering airing ads before its videos, but Harris Interactive discovered nearly three-quarters (73 percent) of frequent YouTube users (again, that’s only 14 percent of the total) say they would visit the site less if YouTube were to start doing so.

Aongus Burke, senior research manager of Harris Interactive’s Media & Entertainment Practice, said of YouTube, “It has really emerged as a major force in, and problem for, the traditional entertainment industry. Not only is YouTube using a lot of their own content to steal the eyeballs they want the most, the site has provided a launching pad to wholly new forms of user-generated video entertainment that are gaining popularity quickly.”