This question can be answered in numerous ways. If we realize that the medium itself is a conveyor of meaning, then the medium is as important as the message. The reason why this point is important to consider is because the medium is not neutral or value free. It has many connotations from society. Certain media convey wealth; others convey youthfulness and still others convey gender. I think you get the picture.
So, if you know the target group, then you an tailor make your message and chose the right medium to reach them. Without considering the medium, you may not reach your intended audience. Keep in mind that the medium is the message. Some good social theory will help you think through the issues.