A few horribly acted individuals aside, Sony's newest teaser trailer shows off not only it's new fancy (tiled...ugh) UI, but also how quickly you can switch between the social aspects of the PS4 and also play a game. While in game, you can switch seamlessly between playing a game (they use Knack in the video), watching a video put up by a friend, and go right back to the gameplay. Also, when getting a voice chat (without getting any prompt seemingly) from a friend, you can go and download the game they are playing, and for example of Killzone, downloading either the single play or multiplayer first, and then download the other part in the background.
Also in the video, shows off what you can do with mobile integration, add friends, talk with them, install games remotely, and a ton more. The video is below, and it is quite impressive, acting aside....