Extreme example that illustrates the point fantastically: 37signals valued at $100 billion after investor buys 0.000000001% of the company for $1. http://signalvnoise.com/posts/1941-press-release-37signals-v...
It does illustrate how these don't mean anyone actually has $10 billion cash[1], but that wasn't the point here.
[1]: which does still confuse many as shown and explained here: http://money.stackexchange.com/questions/11368/how-are-start...