Social media has had the effect of opening up the channels of communication to all of society.
Since the industrial revolution, there had been a tendency towards a one-way system of communication via media such as newspapers, radio and television.
Everyone now seems to have the ability to post content and read what others have posted. Anyone can step up and begin producing content to be consumed by those naturally drawn to it. One has the ability to choose what to consume. To have an audience, all that one needs is to be found via a search engine.
In doing so, it seems to have swept away the mode of one-way communication. In a way, it has brought back the public village square of the pre-modern era, where one presumably came across people discussing things, preaching and ranting.
This also has had some interesting side effects. It has resulted in the hardening of the so called “bubbles” and fractionalization, wherein people of a particular persuasion (political or otherwise) only tend to read content from and associate with others who think similarly. Much has been said about the effect of social media on events such as the Arab Spring. It remains to be seen over the course of the next century or so how the long term historical effects will look like.