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.
I tried to find where I saw the story that some inactive accounts won the weekly lottery... it turns out that some "forgotten accounts" have active referrals that earn them sats, RPs, and of course lottery tickets. At least I remember that, but I can't remember when and where I read it.
Even I remember that conversation. It was said that the freebitco.in account had won the prize but never claimed it or used it.
I guess such accounts which are not utilizing their satoshis directly benefit freebitco.in since they don't have to pay it from their balance.