What Facebook, Flickr, YouTube, Blogger.com really is, is that you are creating a presence for yourself, an extension of you that people can access online.
Marshall McLuhan's seminal book, "Understanding Media: The Extensions of Man" puts forth that mans inventions are extensions of himself.
The phone is an extension of the ear and mouth.
The car is an extension of the feet.
The TV is an extension of the eyes.
Using this metaphor, we can generate an infinite flow of ideas on how we can create services and ideas for internet business startups. (Though success may not necessarily follow.)
Facebook is an extension of our need to express our current feelings, our emotional moods and state which we need to express to our fellow man.
Flickr is like this virtual photo album which we would like show the whole world.
Just go through your day, and jot down the things you do, everything from waking up to talking to people. Which of these activities do you think would be made more efficient or better by web-enabling it?