I think it's not an Annual investment may be it's just another additional source of profit.
because all the money in my account is from roll and REFERRALS
That's my opinion.
What are you talking about?
It doesn't matter if Bitcoin in your Freebitco.in wallet is from faucet, referrals or from deposit.
The only condition to receive 4.08% annual profit is keep in wallet more than 30k satoshi, nothing more.