data
is an object* not an array** so it doesn't have a length property.***
Use a debugger like Firebug, Safari/Chrome dev tools and use this code instead:
$.getJSON("http://twitter.com/users/show.json?screen_name=starbucks&callback=?" ,
function(data) {
console.log(data);
});
and you can see that the data is coming back to you perfectly.
Try this to see what I mean, preferably with a JavaScript console available.
*e.g., something that looks like {key: value, ...}
; also known as a hash or an associative array
**e.g., something that looks like: [foo, bar, baz, ...]
*** unless, of course, someone was evil and constructed the object like so:
data = {
...
length: 8675309,
...
};