While it sounds like good news for YouTube, the increased popularity of online video may soon become the bane of its existence.

According to a new study by Harris Interactive, over two-thirds of U.S. adults have watched a video on YouTube over the last year (that's somewhere around 197 million people). And while YouTube is experiencing growth with older adults, the report says it is nearly ubiquitous with those under 30.

So what's the bad news? As technology advances, online video becomes more accessible, and the study shows, people begin to access longer-form content. What's worse for YouTube is that only 8 percent of people surveyed prefer user-generated or amateur content to TV-based content. With this audience preference for longer-form and TV-based content, TV network sites and companies like Hulu and Joost may soon pose a real threat to YouTube.