I am a Skype user and not a FB user and I have no intentions of ever using the FB product again (I deleted my account around 2 years ago). I wonder how many of those 750 are the same as I.
Facebook reports monthly active users - people who have visited the site in the last 30 days. You (and anyone else who hasn't visited in last 30 days) are not included in that number.
There's also accounts for people's cats, cars, bands, and loads of other silliness. So we can discount that claimed 750m some, but probably in the ballpark of not much more than 10%. And OTOH, perhaps Facebook knows this and already considers account-to-person dilution in figuring its revenue expectations.
I still find some pets and companies using profiles, but most of these are pages these days (since there really aren't advantages to making a profile vs. a page for them), and there is a process for converting these to pages - https://www.facebook.com/help/?page=18918 - and it is the way to reactivate a non-human account that is reported.
Edit: <Joke> At this rate, they'll have more users than there are people on Earth by the end of the year.</Joke>