There are those who would say that modern technology has served to divide us. Where once we interacted only face-to-face, now we interact just as often at a distance. However, the Internet has just as much potential to bring us closer together. One way to do so is through shared online experiences.
Some of the first software to bring us together was the old bulletin board and chat applications. Lately, there has been a rise in a particular type of social application. This type involves in-browser entertainment media -- sometimes interactive, sometimes not -- in which all participants see the same thing at the same time (though sometimes from different perspectives.) A chat accompanies this in the same view and users have fun talking about what they all see taking place in the entertainment media.
Some of these such as Justin.TV and Deep Games allow multiple participants separated physically to watch a single movie or to watch a single person play a video game. Others such as Twitch Plays Pokemon allow the chat participants to control a game collectively via the chat. Kill Brawnkor lets players switch between playing and chatting seamlessly. This simple but powerful formula has an inexplicably strong draw.
I think perhaps the addition of a social element such as a chat to commonly solitary entertainment activities simply combines two fun activities in a complementary way. Also, while experiencing the media, players may discuss the experience with others who are experiencing the same events simultaneously. Finally, the text-based format of the chat allows discussion in a way that doesn't feel like it interrupts the flow of the entertainment experience.
Ultimately, though, it's just plain fun to watch movies and play games with others. I predict we'll continue to see new takes on this "shared experience" mechanic in the future.