PubSubHubbub is a protocol and reference implementation for doing publish-subscribe using web hooks, polling in feeds triggered by a ping from the publisher, and POSTing Atom entries to notify subscribers. The notification part is similar to what I've been working on for the publish-subscribe stuff at Mediamatic Lab, where we spiced up Idavoll with an HTTP interface to bridge the gap between XMPP Publish-Subscribe and HTTP speaking entities.
Although I spend a lot of time working on XMPP based publish-subscribe, I understand the reasons for going for a full HTTP-based approach. XMPP can be intimidating for developers of web applications. While the differences between XMPP and HTTP are important (stateful connections, asynchronous processing, etc), the fact that it is different is reason often enough. Hosting facilities don't always offer ways to do XMPP, and there is not nearly enough running code out there to make it easier for people to play with these technologies to spice up their web application with non-IM XMPP functionality. Having platforms like Google App Engine provide sending and handling raw XMPP stanzas as part of the API would surely help.
That said, PubSubHubbub has two separate sides to it, the publishing part and the notification part. There's nothing that prevents a hub to do the publishing part using regular XMPP publish-subscribe. Instead of fetching the Atom Feed over HTTP every time, it could use autodiscovery to find out the publish-subscribe node and upgrade by subscribing to it instead. Similarly, the notification part could send out XMPP notifications. Combined with existing HTTP aggregator, that combination is very similar to how the aggregator for Mimír works.
I'm still not convinced that PubSubHubbub is the answer to the efficient exchange of updates on social objects, but I do think it is a good way to make smaller entities be part of a federation of social networking sites. Likely, we'll see a hybrid approach, to begin with.