What arguments do you have against a 'pull' style for notifications? Something like a GET to /latest that passes in the last time the client checked for updates and only gets back the URI of the last 10 or so posts since then. An HD video post could contain simply a URI that points to the video, so the video isn't even downloaded until the 'app' within tent asks for it.

This way you could have a path with all the videos on it that you could proxy off to a server with more bandwidth. It would get the user a lot more control over how their content is accessed.

Edit: And what if my server is down when one of my friends makes a post? Will I never see it?

Pull-style requests actually use up quite a bit more bandwidth than push notifications, because every client and server has to query every few seconds instead of just getting notified when there is a new post.

Posts have 'views', so a video or photo post would be pushed out with the 'meta' view which would include a URL to the content instead of the content inline.

I don't think so, considering that not all of my followers are going to be using the service all of the time. If the server just blindly downloaded everything always, then yes, it would be more bandwidth. But with proper caching strategies and servers only requesting new posts when clients are active, I think you could save a lot of bandwidth by not pushing everything.

While the example of Justin Bieber makes it seem to make sense to push the cost of following to the followed rather than the follower, I am not at all sure this makes sense in the general case. It puts the user at the mercy of spammers & bots and ISTM making the follower bear the cost of following would disincentivize this kind of bad behaviour.

What about allow a publisher to delegate a subscription endpoint via something like pubsubhubub? Then publisher just has to push to the aggregator, everybody does their pulls from them.

That only moves the load problem, doesn't solve it - given a decentralised system you're likely to be responsible for load on the aggregator server as well.