Well. There are about 7 billion people on Earth at the moment. Allowing also for the c. 500 million deaths over the last 8 years means that there are 7.5 billion people or so who theoretically could have had a facebook account.
I believe Facebook has 850 million accounts. Some of these won't be active, but even so let's pretend they all are.
so 850,000,000*100/7,500,000,000 = 11.333...
So about 11%.