I remember back in 2005 someone has forwarded me the image with a list of web 2.0 logos. The list contained 450 newly created services. It was amazing, for some reason on that day it felt like Internet was completely different place to what it was before. Everyone wanted to get their slice of Web 2.0.
5 years later, it is clear that there are dominant services that users prefer to use on daily basis and here they are: Facebook, Twitter, YouTube, WordPress. Just 4 out of 450. Why? Because they simple to use and allow users to communicate instantly.