"What are the differences, socially, between singing on the cart coming back from market in 1755 and in the back of charabanc coming back from a works outing to Southport in 1955?

Why do people 'draw the line' where they do?"

Because what most people including Steve Roud, Cecil Sharp, et al, agree on is that for it to be 'folk' it has to have been passed down, not just sung. So technically it would depend not only on what your charabanc passengers were singing (hits of the day, Music Hall, WW1 favourites?), but how they had learned them.

Although it's not a hard and fast 'line', the advent of commercial recorded music and radio represents a watershed as far as this argument goes.

And singing was 'actively practised' as a community activity less and less as the 20th century wore on. Speaking as a 1970s era football fan, I'm always shocked when I go to a match now how the number of people actively taking part of singing has diminished, even though the creativity still flourishes. As to karaoke, although it might be 'folk song' by the broadest definition, the fact that singers read the lyrics from an autocue and perform to a backing track makes it something different to 20 verses of 'Lord Bateman' learned from your grandmother.