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

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.