I have been in the advertising world for about a year and I will admit some days I struggle with it. I am definitely one of the quietest at my company due to being an INFJ and just being plain shy. It is hard for me to speak up in meetings with lots of people, but when I am left to think and work independently I usually produce great results that are noticed by myself and executives. Since INFJs tend to be extremely sensitive to others wants and needs I think our place in the advertising world is vital. Not to mention our unique sense of looking at things from many angles and always striving to be our best. Sometimes I feel like I'm not making a positive impact on society, just telling people to exercise their right to consumerism, which is something I do struggle with being an INFJ. But there can be enough opportunities to insert INFJ's needs to make the world a better place, you just need to know where to look.
I'm not sure if that helps at all, but if advertising is your calling I wouldn't stray away from it!