>>12358807That gif is obviously incomplete, though illustrates the increasing polarization since the 90s. Figures like Newt Gingrich have been blamed for a lot of the growing divide and dirty tactics of the 90s, which have only compounded with the partisan obstructionism of figures like McConnell.
There's been ebbs and flows in polarization and radicalism. Obviously things were extremely volatile around slavery, populist agrarian vs manufacturing interests in the late 1800s, and socialist workers vs capitalists in the early 20th century. The era immediately following WW2 was probably the most tranquil in terms of political harmony, in large part due to national feelings of victory, unity, and thriving economic conditions.
Radicalism increased following Civil Rights movement and Vietnam War. Stagflation ended bipartisan unity over Keynesianism, leading to Reagan neoliberalism. Americans have also been increasingly segregating themselves along partisan lines and the rural-urban divide.
The wiki covers a lot of the contributing causes fairly well. Point is, it's not just social media. People have been radicalized all throughout history, from Peasant Revolts to the French Revolution, Spring of Nations, Tsarist Russia, Weimar Germany, etc. The causes are more complex than filter bubbles.
https://en.wikipedia.org/wiki/Political_polarization#Causes_2 
