For instance, newspaper, radio, and TV all reached unprecedented numbers of people, but they didn't bring the same scale to the number of folks contributing content. To use your Letter to the Editor example, a newspaper only has so much space, of which only a certain portion is allocated to displaying such letters. This means the vast majority of the country could never have their letter published just due to space limitations alone.
Modern social media like Twitter is notably different because it has scaled the input just as much as the output. Every consumer can now be a producer as well. The idea that anyone can be a producer is very powerful and is what makes it feel more like a public space than, say, a newspaper.
It's more like talking at a bar, where others can overhear you. Twitter can of course start throwing people out, but then it turns into a clique and will probably be eventually subsumed by the larger society that it ignores.
Somewhere between "handing out pamphlets in Hyde Park" and Twitter there were plenty of other shifts in communication patterns as well. There was a good 100 years or so where if you wanted to be heard you had to get your message into a newspaper, and then another 50 years where you also had the alternatives of TV and radio. All of these were controlled by corporate interests, and it's not like there was ever a right to have your Letter To The Editor published.