It's true that many of the users aren't active on the site despite having an account.
Even I had previously forgot my email and password to freebitco.in account and had to create a new account.
I guess there will be many people with similar case and so the real active number of users will be far lesser than the number of accounts on the site.
It is an absolutely normal thing when the number of constantly active users of a service or website is much less than the total number of registered users. In the case of freebitcoin, they have several hundred thousand users who are active on a weekly basis, and it looks like a great result to me. Of course, there could be millions of "dead" accounts out of 52 million accounts, and freebitcoin could delete them as some other services do. However, I don't think it makes much sense.