Look at the current online majors: Google, Amazon, Apple, and Facebook.

Google has search intentionality. They benefit from the fact that they're aware of when you're likely to be looking specifically for something. Plus tremendous marketshare in search -- on the order of 80% or so.

Amazon has purchase intentionality. Fewer transactions, but where they are, Amazon has disintermediated advertising because they're fulfilling the transaction. Also tremendous marketshare in commerce. Their margins on sales however are low.

Apple is a slightly different beast: hardware, plus some software/information goods (largely entertainment sales). And no, I haven't peeked at their financials. Profits on the hardware side are tremendous, though marketshare is relatively low. Marketshare on their software/information sales (software, music, video, books, games) is higher, margins tend to be low.

Facebook is in the ads space, and lacks for intentionality though its' got a lot of loiter time. People spend hours on the site (or can). The focus should be less on transactions per page, as on opportunities per user. Focus on detecting when someone's in a purchase state and utilize that, as well, perhaps, as the relationships between users and vendors which can be provided. Of all the majors, I'd say FB's position is the weakest, and the degree to which the user experience seems to be deteriorating under increased conversion pressure seems to support this.

Of possible breakouts:

Google going into direct retail or direct retail partnerships, which seems to be happening with Google Shopping Express, could be a net win. Google's disadvantage is increasingly negative response to its data profiling. Abandoning this for other bases on which to generate revenues would be an interesting move.

Amazon is a latent player in the hardware space. To date I've found its tablet and e-Reader products unappealing because the're too ostentatiously shopping malls thrust before the user. A much, much softer touch would be more successful for me. Jeff Bezos's personality may be the company's Achilles heel. Amazon also have an interactive an online component through their forums (and product ratings) which might be leveragable into social space, though I consider that a very long shot.

Apple could branch into either retail or search. It's risk is getting out-competed in hardware, particularly in the iPhone/iPod space where Android is now eating its lunch. On the other hand, there's an opportunity in that Apple doesn't rely on the intrusive surveillance that Android is predicated on. That pivot is complicated by iOS being a closed rather than open operating system. I see the opportunity being frustrated by Apple's preference to keep its software platforms closed. Its laptop and desktop market could grow at the cost of further decline of Microsoft, though challenged by Chrome devices.

Facebook has the most difficult transition of any of the four. It's not in retail, pushing advertising reduces the appeal of its other offerings, and it doesn't have a hardware offering. Yet. That last could be a breakout, though again, I see this as a long shot.

Of the four, FB is the most likely to fail or decline. Google is probably well-poised to retain king-of-the-hill status. Apple is likely to remain as a center of the personal-electronics / entertainment market, though pinched by Android in small devices. Amazon could be vulnerable to an open-platform based retail system based around or promoted by Google, though it would take a lot to topple it. Expansion into devices would be useful but not a major game changer.

Other disruptive elements: a decentralization of social networking space through open-protocol networks (e.g., Diaspora, Friendica), and self-hosting systems (e.g., FreedomBox). A federation of search, whether through meta-providers (e.g., DDG) or truly federated search (YaCy). An open handset or open hardware spec. All of low to modest probability, but not impossible.