Twitter used to be a protocol - rather like SMTP or IMAP. Other people made Twitter clients, with many different interfaces, and Twitter poked around with metadata (adding retweets, for example), but the user experience was something other people built.
Then Twitter pivoted, and deprecated the third party apps, and took control of the interface. The obvious thing that it did with that was to deliver a predictable offer for advertisers. But the more interesting thing to me was that it created a canvas - which is now turning Twitter from a protocol to a platform.
Twitter is turning 'Twitter cards' into a platform. You can embed video, or slides, or music - all sorts of things. You can embed a call to action that will harvest the account's email address. And, increasingly, you can drive acquisition - of Spotify users, or apps, or customers. And thanks to retweets these cards can end up anywhere on Twitter, far beyond the original poster's network.
Twitter isn't the only company doing this sort of thing. Many of the new crop of social messaging apps, such as Line, Kakao, WeChat and Kik, are also creating canvases of various kinds within their services - within individual messages. Kik and WeChat are exploring HTML5 games within the app itself, and WeChat is playing with retail coupons. Some of the results are pretty strained, but there's obvious value in being able to send tracks, game levels or yelp reviews through such apps, and sending them as rich, actionable cards is much better than a URL string.
(Interestingly, though, probably the biggest social messaging app outside China, Whatsapp, is pretty much the only such app that isn't trying to become a platform in this way, at least not yet.)
Cards are an interaction model that are spreading pretty widely, in fact. They're an important part of how Google presents the newish 'entity' based search, which crop up in the right sidebar on the web and of course as part of Google Now. Part of the magic is the semantic understanding of data that lets Google make these automatically, but the presentation is, again, a card.
And then there's Airdrop, an intriguing feature in the new iOS7 that's been rather buried by all the fuss about the new visual design. Instant, zero-configuration local sharing (remember Bluetooth?). Apple's screenshots focus on photos, but to developers this is just part of the standard sharing API, and you can put anything in here - coupons, game levels, deep links to reviews or songs. But instead of sending it by SMS or email, you can pass it across a bar top. No canvas, this time (except for pictures themselves), but again the atomised content.
What all of these have in common is that they're pulling information out of the app or the service and making it relevant to the moment. They're taking things out of silos, packetising them and making them sharable. But at the same time, they're making them canvases - not just files, but cards, content, real things that you can pass around.
In some senses, this started with Facebook, which had a canvas a long time ago (in internet time, at least). Facebook is present on mobile and doing well, claiming close to 800m mobile users, though it isn't close to winning in the sense that it won on the desktop, not with hundreds of millions of Facebook users choosing also to use WhatsApp et al.
But Facebook is about aggregation - about sucking everything into the gravitational well and spitting it back out through a black box filter to stop you being swamped. Facebook is an endless stream whereas cards are individual. The point of 'cards', like the story of mobile social, is disaggregation - of the over 200m people who already had Facebook but are using WhatsApp for messages - the 100m Instagram users who prefer it to Facebook for photos, and so on, and so on.
From a business point of view, this is interesting because it points to distribution and discovery. How do new products and services get passed around? How does social sharing evolve? Complexity is increased, too - how do you do SEO for Google Now? How should you think about conversion rates on a Kik card or a game shared over Airdrop? There's a sort of inexorability to this: Zawinski's Law states that "Every program attempts to expand until it can read mail." One might now say that every product and service online expands until it can distribute freemium games.
I think there's also a question of being native to the platform, though. Chris Dixon wrote recently about finding things that are native to mobile as opposed to mobile versions of desktop products. What could be more native to a smartphone than a piece of content the size and shape of a smartphone screen, that can be sent anywhere?