“The medium is the message”. Marshall McLuhan’s dictum, which defined the post-Second World War marketing era, has become more pertinent than ever in this age of the worldwide web.

The medium has multiplied. Teenagers, for example, have at least three screens - mobile, laptop and TV. And the message is instant: we poke our Facebook friends, text our mobile mates and Sametime our colleagues (unless we’re Twittering, of course).

McLuhan focused attention on the medium itself rather than its content, which, he asserts, is changed by the very channel through which it is communicated. Society at large, he said, is affected by the characteristics of the medium through which the messages are cast.

Today’s social networking is surely the apotheosis of his thinking. The teenage son of close family friends has recently been abandoned by his girlfriend. He simply changed his status on Facebook from “in a relationship” to “single” and was instantly inundated by messages of comfort and support and, who knows, even more.

In the corporate world, Ocado’s initial public offering last month was briefly overshadowed by the news that Amazon has moved into groceries (or ‘messages’ as, ironically, they are known in Scotland).

In the pure-play versus multichannel debate that currently animates the analysts, the media seems to matter even more than the messages. And Amazon’s other recent announcement, that ebook sales are exceeding those of hardbacks, is the best example yet of McLuhan’s adage: the medium is online and now, so too, is the message itself.

Amazon’s fellow giant in the internet age, eBay, is also diversifying, on a scale and at a pace that dwarfs most of its rivals. PayPal, its online payment service, now operates in 24 currencies and 190 markets. And eBay’s other specialised divisions, StubHub (its ticket marketplace) and eBay Classifieds, together have a presence in over 1,000 cities around the world. The online auctions business has driven eBay’s absolute supremacy in web hits (with a 21.4% share of UK shopping site visits last week, according to Experian). But eBay is moving rapidly beyond these roots to a fixed-price marketplace. More than half of its listings today are new products, offered on a ‘buy now’ basis.

Despite its recent moves into fashion and the well-publicised success of its Outlet business, the Big Story in last week’s Retail Week, ‘Putting media at the heart of selling fashion’ curiously made no mention of eBay, although big brands like Superdry and Nike have signed up.

The launch of a fashion app for the iPhone secures this as a key category for eBay. It is introducing consumers to a new way of buying clothes: assembling bespoke outfits, trying them on in a virtual dressing room and canvassing the opinions of their social networks.

As smartphones and tablets proliferate around the world, mobile-commerce is forecast to be a $119bn global industry by 2015.

So, if the medium is the message, then, surely, the message has to be mobile.

Michael Poynor managing director, Retail Expertise