The post-war partisan environment was pretty unique in American history for its civility and substantial overlap between the two parties. The late 19th century had a level of partisanship comparable to today, though much of it was filtered through ethnic and religious identities in a way that would be unfamiliar to people today. In many ways, our modern political environment is a reversion to the mean. I wouldn’t bet money on social media specifically being the primary driver of this change (though I do think the collapse in barriers to sharing information has allowed for partisanship identity to be formed along ideological lines, rather than geographical or cultural lines, in a way that is unique.)