JSON or XML, questions about an idea ?
TomOfTTB: My first question would be "what are you writing this social network in?"  Because I think every mainstream language in existence has free libraries to create both XML and JSON output (Javascript is made for it, .Net has WCF, there are several Gems for Ruby, php has _encode, etc...).  So implementing both should just be a matter of passing the values to the appropriate function and reaping the benefits of already available libraries.That in mind I'd say both.  If for some reason you insist on choosing I'd probably say JSON.  XML's only real advantage over JSON, imho, is it's human readability.  That makes it great for stuff like configuration files.  But for machine to machine communication JSON is faster and less prone to conversion errors.  Plus it's easier to accept JSON in Javascript which you defined as your target audience.