It is an interesting debate. I was never quite sure when the media lovingly pushed the term 'United' over to the Mancs. I guess the advent of social media in general has helped. The press/newspapers have done it for a long time. It irritated me back in the early 80s when reading match reports.
When I have football conversations at work with some Gooners (I'm in London), they use the term to describe those cunts. This includes old school supporters and modern day new age Premiership supporters, but they're not the passionate supporters who travel home and away, still annoying though. I get mildly irritated and slip out of the conversation.
We used to sing 'United' back in the day, but you rarely hear it nowadays.