America’s Funniest Home Videos may have pioneered the YouTube concept, but as the site reaches the five-year mark, its audience size is no laughing matter. YouTube’s viewership now exceeds that of all three networks combined during their “primetime” evening time slot, with more than 2 billion views per day, Google announced Sunday.

Granted, YouTube’s numbers come from worldwide views, while ABC, CBS and NBC broadcast their primetime channels within the United States. But this is a significant milestone nonetheless, and hints at an eventual tipping point when the internet could become the world’s dominant video-delivery system, Mark Cuban’s predictions aside.

Google also trumpeted some other key stats: People upload over a day’s worth of video to YouTube every minute; the average user spends only 15 minutes a day on the site, which YouTube would like to increase in part by renting full-length films; and YouTube has broadcast live sports to more than 200 countries.

To celebrate its fifth birthday, YouTube asks the site’s users to upload videos of how the site has affected their lives, some of which will appear on a specially curated channel. In addition, celebrities including Conan O’Brien — whose best next career move might be to become official curator of YouTube — marked the occasion by posting a playlist consisting of their favorite videos (view his above).

Should the networks really be worried about being overtaken by YouTube? Yes and no. They own their content, YouTube has professed a wish to lengthen viewing times. Licensing currently-airing full-length network television shows (in addition to the older shows they currently license) would be a great way to do that. And the networks are in a more favorable negotiating position than the record labels were when they made similar deals, due to Hulu (ABC and NBC) and CBS.com already attracting large audiences for that content.

Perhaps a more serious threat to the networks is that YouTube is changing our viewing behavior, and that our viewing habits on the computer will soon migrate to the living room.

Plenty of set-top boxes already play high-definition and even 3-D YouTube videos on a television set. When Google unveils its next-generation set-top box, possibly as soon as Wednesday’s I/O Conference, in partnership with DishNetwork, Intel and/or Sony, YouTube will assume an even greater presence on the television. Even if the networks continue to hold back their full episodes of new shows from on YouTube, users could come to prefer a higher percentage of direct-to-internet content on their televisions.

As paidContent founder and editor Rafat Ali tweeted Monday morning, Conan O’Brien seems “a lot funnier on the internets” than he did on network television, and O’Brien recently joked with a roomful of Google employees about a world without television networks. Who knows, five years from now, O’Brien could be hosting his own show on YouTube, rather than fretting about his terminated NBC contract.

“I don’t know what television’s going to be five years from now. There’s a lot of people that think you’re just going to experience it all through your server, and people don’t even know how the business is going to change,” said O’Brien, who should know, as a longtime television host and writer-producer of the Simpsons.

“There might not be really network television as we know it — wouldn’t that be sweet.”