Watching TV has always been a social experience, although today’s users are more likely to post about the big game on Twitter than to gather with family on the couch. Startups and established companies alike are developing new ways to take advantage of this social side to watching video.

Last week, Facebook launched a live streaming application that plugs into its social network and lets viewers interact with each other, and content producers, while watching. Several other companies are developing similar offerings that they hope will keep users around longer, and help advertisers target groups with particular interests.

A New York startup called Qlipso already goes further than Facebook. It offers a range of social features including voice and video chat, and 3-D avatars that let users make gestures while watching online video. A “virtual economy” will also reward users for participation with virtual cash that can be used to buy items for their avatars, such as articles of clothing.

Earlier this year, Qlipso bought the online video site Veoh and its audience of roughly 13 million unique viewers a month. (A lawsuit with Universal Music Group had drained Veoh’s finances, even though the startup won the legal battle.) Qlipso CEO Jon Goldman says that ever since Qlipso’s social features were integrated into Veoh, users have been spending twice as much time watching videos–an average of 20 minutes per session, up from 10 minutes per session.

More established companies are working on similar offerings. Tunerfish, Comcast Interactive Media’s social discovery engine, lets users share information about what they’re watching and what they think of it before, during, and after a show. And YouTube has experimented with features that let users see who is watching a video and chat with them.

Goldman says Qlipso aims to replicate online what people do naturally around television content, while making it easy for a mainstream audience to participate without having to download extra software or learn new skills. Goldman, who has a background in the video game industry, relied heavily on that experience to direct Qlipso’s design. After seeing how engaged and loyal users get in social gaming environments such as World of Warcraft, Goldman wanted to apply that to a more casual experience. The use of avatars and voice chat, for example, were both inspired by the belief that these technologies make users more loyal to a game community.

“The key thing is figuring out how to do it all in the browser,” Goldman says. Qlipso runs a 3-D animation engine on top of its flash video, meaning that users don’t have to download any software to use a 3-D avatar. The company also has to be sure that users who are interacting have synchronized video streams, so that their comments make sense in context. The company has found that viewing groups form based either on real-life friendships, or shared interests.

Goldman says that a more social approach to video provides an attractive target for advertisers, who know they can reach engaged users with a common interest. The company plans to extend this vision by starting “celebrity rooms,” where larger audiences can interact in special viewing events hosted by celebrities or bands.

John McCrea, general manager of Tunerfish, says that social Web technologies lend themselves to being integrated with other sites. This helps new sites become part of a social ecosystem by drawing on connections made on other social sites, including Facebook.

Steve Rubel, who watches emerging technologies and trends as senior vice president and director of insights for Edelman Digital, notes that people already talk on Twitter during TV events such as the Soccer World Cup. Rubel believes the trend can definitely benefit advertisers, provided they’re careful to coordinate their social media campaigns with ads they run on social video sites.

But Marie-Jose Montpetit, an invited scientist at MIT’s Research Lab for Electronics who has studied social TV for several years, warns that, while social features can certainly be valuable when integrated with viewing technologies, companies need to remember that users will have different ways of interacting around different types of content. For example, they might love to use voice chat while watching comedy or sports, but could be irritated by it when watching a drama.

Montpetit says companies also have to walk a fine line when trying to create affinities between users. For example, when companies have introduced chat functionality, she says, some users have reacted negatively, saying they don’t care about the opinions of random strangers.

Traditional television poses a different technology challenge for those who want to add social features, Montpetit says. Ultimately, however, she believes users will get the same highly connected social experience as Web video. She believes that users and content creators will eventually stop making strong distinctions between Web content and traditional TV content. “All these screens are becoming the same,” she says.