All of your sources are actually saying basically the same thing.
The Estates are areas of power in society. Originally and for a very long time, political power and religious power dominated the lives of every person in the world. The Third Estate, the middle class came about a few hundred years ago. Before that, it was just government and the church who held power, but now regular people mattered because they could act collectively with power independent of government or church.
The Fourth Estate came about a bit later with the development of the printing press. "The press" was a term used for people who were journalists and who sought to find things out and print them for others' benefit. With the advent of mass media, "the press" has arguably become as much of an institution as religion or politics. It certainly has the power to sway the course of events, and thus is considered yet another Estate.
Anyone who participates in society from a fringe or "outsider" position is the Fifth Estate. This role in society has mushroomed since the inception of the internet as all of us are now able to share information more or less freely and with relative ease. The Fifth Estate refers to anyone who attempts to use the power of instant, widespread communication to influence society.
Notable examples of this type are bloggers, watchdogs, activists, or other individuals or small groups who track and comment on the first four estates: government, religion, business, media.