It's a very elusive term, in relation to the progressives - as even in today's Western societies, while they are more liberal than ever, those people still complain about Social Norms somehow troubling downtrodden demographics;
A social norm by itself is just a mode of behaviour which is most common within a society. However, this does not prove it's quality - as
@ssaneenah noted in his own post, beliefs like Christianity have strong positive moral positions, which differ from the wordly (I.E societal) norms. Christ arguing against the pharisees is a very good example, in a religious context
The nature of the Progressive ideology is that it does not stop - even if a certain result they planned is reached, due to their delusional ideas, it is not enough; Therefore more must be done, and even more progressively than previously. Which means that even if the "Social Norms" become predominantely progressive, the progressives themselve will then find issues with them and demand the Overton Window to be pushed wider.
Essentially Social Norms for those people are the same as anyone, but the difference is that progressives do not draw a line for themselves; They do it for the opposition (Traditionalists, Conservatives, other ideologies), outlining what is offensive\unacceptable and what is not, and eventually consider outdated their own ideas which have outlived their usefulness.