Ooh! Here's one I can answer:

The use of the red flag actually goes back earlier, to the French Revolution(and earlier). In piracy times it was used to signal a fight to the death, a sort of "go away or we'll have to kill you."

A red flag hung at the Champs-de-Mars was used as a royal order to disperse, after which troops were ordered to fire into the crowd. The Jacobins turned that around during the Reign of Terror, using red flags to symbolize the blood of revolutionary martyrs. In the flourishing style of Thomas Carlyle:

This is that famed Martial law, with its Red Flag, its 'Drapeau Rouge:' in virtue of which Mayor Bailly, or any Mayor, has but henceforth to hang out that new Oriflamme of his; then to read or mumble something about the King's peace; and, after certain pauses, serve any undispersing Assemblage with musket-shot, or whatever shot will disperse it. A decisive Law; and most just on one proviso: that all Patrollotism be of God, and all mob-assembling be of the Devil;—otherwise not so just. Mayor Bailly be unwilling to use it! Hang not out that new Oriflamme, flame not of gold but of the want of gold! The thrice-blessed Revolution is done, thou thinkest? If so it will be well with thee.

After the restoration, the red flag was kept by the radicals to show their indifference to the monarchy, which had coopted the Tricolore and some other revolutionary emblems. By 1848 it was emphatically the emblem of republicanism and used at the barricades both against the monarchy and the Second Empire. Victor Hugo has the rebels of 1830 using a red flag in Les Miserables, but it seems possible that he was merely projecting back from 1848.

Incidentally, 1848 is also the same year Marx and Engels wrote the Communist Manifesto. Socialism shares a lot of the same geneology as republicanism, but it wasn't until 1871 that a socialist government, the Paris Commune, actually used a red flag.