The total number of users on Facebook is not publicly available data, and in fact would be extraordinarily difficult to determine given the way large databases work.

Facebook does however post a number of statistics about themselves on their marketing site, which is probably the most reputable source you can get unless you want to directly access the API.

https://newsroom.fb.com/Key-Facts

Thank you Slater. I got the statistics from the link you sent but what about the API ?? Is there a special API ?

Not a special one, just in the standard set of APIs. Here's a link:

https://developers.facebook.com/docs/reference/apis/

You're probably going to be most interested in the FQL api.