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When designing and some new IM based services two concepts of interoperability are available:

• try to offer your service to most IM systems
• chose an open protocol, that's to say XMPP, and stick to it

Apparently the first solution seems more portable. Try to imagine a small startup like ours seeking for venture: the first question in the interview could be "how large is your market?". You sum the figures of MSN, AIM, Yahoo GTalk and others and you get the impressive number of few hundred millions. Impressive uh?  You say "I'm using XMPP, GTalk and few others use it" and you have almost failed the entry question, since your figures lose at least a zero. 

The problem is that this kind of interoperability is the dark side of the force: don't be too greedy with the figures, but choose a solution that can grow, because you may end up with a large unsatisfied user base not using your service. 

Let's explain it with an example. In the last days I've tested the IM integration of Twitter and Jaiku and today I've tried Plugoo , a flash based widget embedding a chat window into a web page and allowing visitors to chat with the page owner. Twitter and Plugoo are interoperable accordingly to the first concept: they allow you chosing any IM system. Jaiku, instead is committed to XMPP only. All these systems have few usability issues, since IM integration is new and there are no well known patterns for solving problems. However the more promising is Jaiku: many of its issues are being addressed and the developers have plenty of options for improving the service (multi user chat for channels, a savvier use of resources, x-html, ad hoc commands, client capabilities, just to list some features of XMPP that may be handy). Instead I don't see many chances for improving Twitter IM integration and Plugoo: apparently open, they are stuck to the small (almost null) subset of common features of the adopted IM services. Any improvement is stopped, but simply (in term of of expressiveness) and cumbersome (for users' interaction) text based commands, that's to say they are forced to used '90s like solutions when ICQ and IRC were already there.

So, when asked "how many users?", just say "one day as many as Internet users, XMPP will be as ubiquitous as now SMTP and the Web are. Why? Because it's the only way to make things work, the other will simply fail" ;)