You are misunderstanding me. Yes the issue is exactly about where to draw the line. The parent's dismissed statements that facebook is a monopoly by saying "you can make every company a monopoly if you make the category narrow enough". My argument is exactly what you are saying, you can not dismiss the statement "facebook is a monopoly" by saying "every company is a monopoly if we make a narrow enough category", you need to actually make arguments (and convince the judge) why the category should be this narrow/broad.