A comment I posted to the comments link that Dave provided (slightly edited):
Saying that the only way to get content is to have aggregators pulling it is missing a whole other way of doing it, that’s proven itself as scalable and resilient over the years: NNTP
I’m not talking about using NNTP directly but taking ideas from it. Having a network of servers that talk to each other, where items propagate between servers, and where users talk to a single server that’s close to them instead of reaching all over the world to poll, makes a lot more sense.
Rather than having a million aggregators hitting 20 or 30 sites an hour each, those million aggregators would talk to an ?RSS server?, using a yet-to-be-designed protocol that borrows heavily from NNTP, and the RSS Servers would talk to each other, so that your aggregator client would simply talk to an RSS Server near you to pick up all the things you subscribed to.
I post a new article on my site, my weblog software sends my RSS item to the RSS Server I’ve configured it for, and from there it propagates. Someone in Topeka wants to read my weblog, they read it from whatever RSS Server they’re configured to use, because my item is there waiting for them. This is how Usenet works, and it works really well.
And there’s no polling of the various servers involved involved – the aggregator connects and fetches all the new items when the user is ready to read them. (With RSS today, if you go away for a few days and you didn’t leave your computer on and polling, you probably missed items as they scrolled off the site’s main feed).
If email was implemented the way RSS is implemented, then my mail client would be polling all my friends every half hour to see if they’ve sent me an email. Nobody would implement email that way – but that’s what RSS is today.
SMTP moves mail from the author to a server close to the user, in the background, maybe involving multiple servers in between. Long term, I think this is going to have to happen with RSS as well.