XMPP is extensible, so the answer could be 'yes'. However, the actual answers are 1. Yes 2. Yes 3. Potentially 4. Yes 5. Potentially Instead of having central

XMPP is extensible, so the answer could be 'yes'. 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.