It seems likely to me that Facebook and Twitter will eventually be competing with Apple in terms of App stores. Facebook sort of already is with their extensive apps platform, however that’s just competing for developer attention. Twitter doesn’t really have an equivalent today (developers mainly build clients and interact with data), but don’t underestimate their clout.
The reason I say this is that Facebook and Twitter have become identity gatekeepers on the net. Already you can login to many sites via accounts with one of the two sites. Creating the API’s to handle purchase/subscriptions and transparently handling the billing to effectively turning a HTML5 site into an “app” is the next logical step. They could undercut Apple and still walk away with a handsome profit for not doing terribly much more than leveraging their size and reach. These apps would work on any device with a web browser. Desktop or mobile.
Given both sites need to diversify revenue streams (something Google never figured out), it seems only logical to make this step. $0.99 for Angry Birds seems more than plausible.
And yes, there are offline abilities in a browser.