Duplicates are a massive problem and definitely account for a higher percentage than they are letting on. Accounts for pets is just the tip of the iceberg, but they won't want to let on as it will tank the share price even more.
Accounts in celebrities names
Accounts in pets names
Accounts for cheating on partners
Accounts for games so you can give your main account bonuses faster
Accounts for businesses who don't like the standard pages facebook let you create
Accounts for idiots who have locked themselves out of their original account
I only use the site to keep in touch with family and friends who are far afield, but more and more obviously fake or duplicate accounts keep cropping up on people you may know. More than actual human accounts. It's a massive problem it's got to be in the 30 to 40% mark.
Also I wonder how they classify 'closed accounts' because when you close it you can log in four months later and everything is there as if you never left it, so are they classing those accounts in their total user base?