As someone who has followed the web and technology landscape for the last 15 years, I have noticed that the companies which have proven dominant are those that demonstrate what I would call a network mindset. By which I mean, they grok the emergent properties of network effects, and use that to establish a dominant position that is remarkably hard to replicate.
For example, Amazon has, from the beginning, tracked user behavior in order to 'get smarter,' providing recommendations based on not just your purchase behavior, but the behavior of people like you. Google's key insight was leveraging the connections people made between web pages (links) and extrapolating importance and relevance. Facebook has utilized the “social graph” to create a service with more uniquely identified users than anything else on the planet. The only reason eBay is even still around today is the genius of the idea of connecting buyer and seller over the network, creating a marketplace that no one has yet been quite able to topple.
One company has conspicuously stood apart from the others: Apple. While Apple was the first company to bring computer networking to the “rest of us,” they've been surprisingly disinterested in leveraging emergent network effects in their products and services. They're attempts at engaging social behavior (Ping and GameCenter) are laughable. The iTunes Store doesn't seem to learn the way that Amazon does. Apple's success has been delivering products that delight, but have never had a product that adapts and learns and grows with you.
Until Siri. Siri is Apple's first legitimate and big play employing a network mindset to drive their offering. Siri learns. It learns from the user, and it also learns through the umpteen APIs that it exploits to figure out the answers to your questions (you can read about the mindset behind Siri in Tom Gruber's presentation. Siri might have changed since being brought into Apple, but it seems to still rely on the same set of connections.)
If Apple is able to exploit emergent network effects the way that the other dominant players have, and combine that with their unparalleled user experience, well, watch out.