When you're talking political polarization on a societal level, it's a distinction without a difference. The distinction is that when you personalize by site, it's an explicit coarse-grained choice by the individual. When you personalize by algorithm, it's an implicit fine-grained choice by the individual, which is then aggregated and turned into an explicit stream of content by the algorithm. In both cases, you are adapting the content someone sees by what they want to see.

The sort of national unity that we took for granted between 1945-1995 (and to some extent from 1920-1945) was explicitly manufactured by mass media. Here technology presented the same content to all people, because it was cheaper to do it that way. Before the invention of consumer radios in 1920, we had a similar political polarization as we do today, if not even moreso. Also the Federal government was significantly weaker, lacking even powers like direct taxation, let alone the mass surveillance it has now.