The question is, how did the Chinese immigration know? for each applicant do select count(*) from people_who_pissed_us where social_username = applicant.fb.username?
It's a high-tech surveillance state presiding over 1.5 billion people with a history of crushing (quite literally, sadly) or 'disappearing' any signs of dissent.
Anecdotally: I once partnered with a Chinese cultural institute for a big event and ended up having my Gmail hacked as part of the compensation. They gotta know you're not a Tibetan separatist after all!