XMPP is probably the best approach. I'm currently investigating this path myself. The disparate set of alternative protocols that app.net is considering are the same ones that diaspora* used. The result is a server that speaks several overlapping protocols.
In order to be successful, what this space really needs is a de facto server, like Sendmail, but one that's easy to set up and configure. Over time, this would make it possible to have a critical mass of compatible servers that speak the same set of XEP extensions. There are many open source XMPP servers. I think one reason this hasn't happened already is that most of them are difficult to set up and configure.
Does XMPP allow you to follow a person and see all their public messages as they're posted? Does it allow you to see historic posts for a user? Can you search the network for all messages with a given tag, and get a live stream of updates? Can you see trending tags?
However, the actual answers are
1. Yes
2. Yes
3. Potentially
4. Yes
5. Potentially
Instead of having central aggregation, the free market would drive sites aggregating users messages to improve their features. You could sign up by just adding the site to your list.
Of course, it would require some work and a draft standard. I'm not saying it's perfect. Apparently identi.ca has a federation system too.
Sure, really we'd be better off replacing them all with something designed along the lines of XMPP but by a modern committee focusing on a few limited features.
The idea of it being a single non profit site seems terrible in comparison.
Why does nobody seem to think of this?