now that phones are finally becoming PDAs, a social network for these devices could provide value. Facebook has an iPhone client that I haven't seen yet. I go back and forth between a webapp that is sized for a PDA and a native PDA app.
The playstation 3 was supposed to come with its own 3D world/social network that any PS3 owner could be a part of just by owning the box. Im not sure what happened to that project, I thought it was an awesome idea. The Wii Neighborhood app creates a plaza on your Wii and you can invite you friends' Miis to show up when they have their box turned on.