Those figures are difficult to come by but there is a site that does a lot of research in getting some of those numbers.
Check out this page: http://www.comichron.com/yearlycomicssales.html
Now we know that the first FF comic came out in 1961 but even in 1962, it still doesn't show up.
In the old days when some people got mail subscriptions in addition to newstand and other retail outlets being the main outlets for buying comics, the Post Office used to require a kind of circulation statement to be printed once a year in the comics. I could go scan one at home.
The FF sales are missing because according to the researchers at Comichron, Marvel didn't start putting those statements in the Fantastic Four until a couple of years into the run.
The first figures for Fantastic Four and Amazing Spider-Man start to appear in 1966 Link: http://www.comichron.com/yearlycomicssales/postaldata/1966.html
Marvel's top sellers are ASM, the FF and Thor in that order. So by that time, Spidey had already grown more popular than the FF.
I don't know if I've ever found a sales ranking from those early Marvel years like we get today from Diamond Distributors. This site has been working with getting the numbers from inside the comics themselves.
Thank you!!! That website is fascinating for so many reasons!
Funny, I always thought FF was #1 faster and longer...