This would be good to note in the post, since right now it's easy to decide not to click anything because there are so many links and the salient feature is never mentioned prior to visiting them!
Depends. Just like SMTP, XMPP can be centralized (if everyone's on the same server, say gmail.com), as much as it can be distributed (if everyone's on his own server) and anywhere in between. Tox can only be distributed.
If it's fully distributed and doesn't use any central servers then user has to bootstrap to network by giving known node information. It's classic annoyance, so having a bunch of bootstrap servers is a good idea. Is that defined as centralized or not, is another question.
