I don't know much about this but from the limited amount of I've read it is probably only a portion of the equity owned, and generally when borrowing against an asset the lender will not give you 100% of that assets value to protect from downside risk. Another probability is that it was adjusted in the past, potentially year(s) ago, and FB's stock price a year ago was almost $100 a share less than current so a $10 drop is not a big deal in the long term.

You raise an interesting question though and I'd like to know the answer as well!