Viacom has succeeded in winning the deletion of some of its content, but many popular clips are still easily available.

That fact underscored the challenges in policing one of the Internet’s most popular sites, where content is supplied by an army of fans eager to share that hilarious bit from last week’s “Saturday Night Live” or “Family Guy.”

Many popular clips on YouTube have come from shows that air on Viacom’s family of television stations, which include Comedy Central, Nickelodeon and MTV.

Viacom said nearly 160,000 unauthorized clips had been uploaded onto YouTube and viewed more than 1.5 billion times.

YouTube’s video swappers have poached particularly heavily from Comedy Central, thanks to programs such as “The Daily Show with Jon Stewart,” “South Park” and “Robot Chicken,” which all feature bite-sized skits that lend themselves easily to sharing over the Web.

Other shows appearing regularly on YouTube include NBC’s long-running comedy sketch show “Saturday Night Live” and “Family Guy,” an over-the-top animated comedy that airs on News Corp.’s (NYSE:NWSA – news) Fox network.

Calls to pull videos from YouTube are reminiscent of the music industry’s largely unsuccessful attempts to stamp out peer-to-peer song sharing services. If a video is deleted, the person who posted it or copied it can make a few changes to the file and repost it.

On Tuesday afternoon, a search on YouTube for “daily show” turned up 2,800 clips, but only a handful actually featured Stewart’s program, with the rest being commentaries or unrelated videos using those keywords to lure viewers.

Similar searches for the network’s crude animated hit “South Park” and faux-conservative talk show “The Colbert Report” showed that nearly all actual footage of those shows had been taken down.

But a search of Fox’s “Family Guy” showed 18,000 results, with the top listings being actual clips from the show running from 35 seconds to more than seven minutes.

A YouTube search of “Saturday Night Live” turned up nearly 5,200 clips, with the top results including a sexually themed Christmas song by pop star Justin Timberlake that had racked up 18 million views.

NBC, majority owned by General Electric Co., has allowed YouTube to host a limited number of clips from “SNL” to promote the show, but in general has been critical of the site. Just over a year ago, NBC asked YouTube to remove an “SNL” rap music parody when it triggered an Internet craze and cemented YouTube’s reputation as the clearinghouse for cool videos.

As the effects of Viacom’s lawsuit worked their way through YouTube, some users voiced their displeasure at what they saw as heavy-handed tactics by — what else? — posting videos on YouTube.

“Let us send them a message that, while they own their content, they don’t own their viewers,” read one message that scrolled into the background like the opening of a “Star Wars” movie. “Boycott Viacom until they get the message.”