That response is a Map, with a single element with key '212315952136472'. There's no 'data' key in the Map. If you want to loop through all entries, use something like this:
JSONObject userJson = JSON.parse(jsonResponse)
userJson.each { id, data -> println data.link }
If you know it's a single-element Map then you can directly access the link
:
def data = userJson.values().iterator().next()
String link = data.link
And if you knew the id (e.g. if you used it to make the request) then you can access the value more concisely:
String id = '212315952136472'
...
String link = userJson[id].link