There are a number of different ways of looking at the final sentence. In keeping with the title, the unnamed narrator is invisible throughout to those who patronize, insult, and oppress him. Like many African Americans in society, his experiences of a hostile world are at best ignored and at worst treated with contempt.
Yet the narrator still has a voice. And he still needs to use that voice to communicate the full range and depth of his life experience. But most people, never having gone through the narrator's experience, will never truly comprehend what he has to say. His message, then, is subtle and clandestine. It operates on a lower frequency that will only be picked up if we attune ourselves to it, if we listen carefully and imagine ourselves in the shoes of society's invisible.
On a specific and immediate level, the final sentence of Invisible Man is of course addressed to those who've endured a similar degree of prejudice and racial intolerance in their lives. At the same time, in an increasingly atomized society in which more and more people become "invisible" for one reason or another, it can also speak to anyone who feels themself cut adrift from their fellow human beings.