Unfortunately, Wikipedia has it very wrong. They've published someone's (the popular) misinterpretation of what Marshall McLuhan meant when he stated (actually as early as 1958) 'the medium is the message'.
To understand what that phrase means, your best bet is to turn to 'Understanding Media: The Extensions of Man' where McLuhan explains (in the beginning of the first chapter):
"In a culture like ours, long accustomed to splitting and dividing all things as a means of control, it is sometimes a bit of a shock to be reminded that, in operational and practical fact, the medium is the message. This is merely to say that the personal and social consequences of any medium-- that is, of any extension of ourselves -- result from the new scale that is introduced into our affairs by each extension of ourselves, or by any new technology."

Nothing there about the medium affecting the content, or message. He's saying that the important thing to note about a medium (its message) is not the content it carries, but the effect this medium has on us, on society.

Respectfully,

Andrew