Julian Assange is the editor and founder of WikiLeaks, and the movie revolves around him (played by Cumberbatch).
The phrase “the fifth estate” plays on a similar phrase, “the fourth estate,” which refers to the press and the news media. Many people have no understanding of the origin of “the fourth estate.” It lies in pre-revolutionary France, when the Estates-General (états généraux) was the (mostly powerless) legislative body of the nation. The Estates-General consisted of three estates (or, more properly, states): the clergy, the nobility, and the bourgeoisie (or the commoners). Referring to the fourth estate was a way of acknowledging the power of the press in affairs of state.
“A Fourth Estate, of Able Editors, springs up; increases and multiplies; irrepressible, incalculable. New Printers, new Journals, and ever new (so prurient is the world), let our Three Hundred curb and consolidate as they can! Loustalot, under the wing of Prudhomme dull-blustering Printer, edits weekly his Revolutions de Paris; in an acrid, emphatic manner. Acrid, corrosive, as the spirit of sloes and copperas, is Marat, Friend of the People; struck already with the fact that the National Assembly, so full of Aristocrats, ‘can do nothing,’ except dissolve itself, and make way for a better; that the Townhall Representatives are little other than babblers and imbeciles, if not even knaves.” (Thomas Carlyle, 1837)
The quotation above comes from The French Revolution: A History, by Thomas Carlyle. It is not a very readable book, having been written in an exuberant and over-wrought style that was apparently very popular at that time. This quotation is often cited as the origin of the phrase “the fourth estate.”
“When the King decided that this hall was a suitable meeting place for the Estates-General, he called in carpenters and painters. They hung the place with velvet and tassels, knocked up some imitation columns and splashed around some gold paint. It was passably splendid, and it was cheap. There were seats to the right and left of the throne for the First and Second Estates; the Commons were to occupy an inadequate number of hard wooden benches at the back.” (Hilary Mantel, 1993)
In her novel A Place of Greater Safety, Mantel paints a rich picture of Paris before and during the Terror. Here (p. 166) the Estates-General are meeting for the first time since 1614, as King Louis XVI tries to pretend he is interested in the will of the people.