Although NATO is unwilling to discuss the matter further, red-faced as they probably are, it's been learned that in classified briefings, diplomats and military officials were told the evidence pointed to the activity being the action of "state-sponsored individuals in China" (i.e., Chinese spies).
In the wake of the incident, NATO has advised senior officers and officials to open their own social networking accounts to preclude another such event. Stavridis now has his own an official Facebook account and the bogus one has been deleted. Facebook said, "After the profile was reported to us, it was taken down as soon as we were notified and investigated the issue."
The incident occurred in late 2011, but NATO officials first confirmed the incident on Saturday.
Notably, officials at SHAPE, the Supreme Headquarters Allied Powers Europe, while admitting the incident, said that "Discussions/chats/postings on Facebook are of course only about unclassified topics."