Manchester United supporters should never refer to the club as "Man U," because it is derogatory insult towards the club that they support.
The term is only used by opposing supporters of other rival clubs as a complete and utter insult to Manchester United. The "U" is meant to be "you" by the rival fans.
An early example of its usage is this chant by West Bromwich Albion fans: "Duncan Edwards is manure, rotting in his grave, man you are manure - rotting in your grave."
As you can see the origin of "Man U" is from a song to insult the dead Duncan Edwards, who was one of the greatest ever United players.
Liverpool and Leeds United fans have copied this with their own "man you" / "man u" versions to insult all of the legends who died in the Munich Air Distaster.
"Man U, Man U went on a plane Man U, Man U never came back again" and "Man U Never Intended Coming Home" (if you combine the first letter of each word you get the word "MUnich").
This makes it a lot clearer that saying "Man U" is an insult, particularly to the older supporters and to see United fans using it now is shameful.
We cannot and do not expect much from the supporters of our rivals, however, we do expect to not have supporters of Manchester United refer to the club as "Man U," because it is completely reckless and shows their complete ignorance.
If a supporter sees or hears that derogatory term used to, they are expected to reach out and educate them as to why not to, and tell them the proper nicknames that Manchester United is called.
Manchester United are nicknamed "United," "Man United," "Man Utd," "MUFC" or the "Red Devils." The term 'Utd' is the proper abbreviation for the world United and nothing else should be used or substituted.
To further prove this point, the official Manchester United website - www.manutd.com - has a "Brand Protection" page and the term, "Man U" is not listed on the page, while the previously mentioned abbreviated terms are. This clearly shows that even the club, itself, does not want to be referred to as "Man U," either.
If you are a true Manchester United supporter, please spread the word, because remember, we are MANCHESTER UNITED!