[Update: before you get too far into these numbers, you might like to check out this updated post, which gives a more accurate breakdown of genders - the country totals remain the same]
Out of interest I went through the Facebook ad platform to pull together some stats on relative numbers of members from each country. Not surprisingly, the US represents over 40% of the total membership at just over 18 million. Some surprises though (at least to me), with Turkey in 5th place.
There’s quite a heavy bias to the ladies too, with female members making up just over 63% of the total population.
I’m not able to see what criteria FB uses to present these numbers, so these may be members with just a basic profile, or those who have also filled in some additional personal details too.
Full picture is below. Click on the image to open up so that you can see all the data.
[update - see my comment in the comments too :-) ]