the annual interest rate is quite good at 4.08% and is given daily, but I wonder how does the site generate enough revenue to give out all the interest?
Annual interest is 4.08% which is something around 0.011% daily, if you invest you will reach ROI in more than 20 years
, but good side is that you can withdraw your money in any time(i hope).
Now, if OP is doing what he claim he is doing, you can't look at interest only as a return from revenue from ads, there is lottery, dice and mining(IMO).
I think the admin wanted people to deposit big amount of Bitcoin into his site as investment, the reason is to cover the bankroll for his dice site.
Not only for dice bankroll, this is from FAQ:
When you hold money in your FreeBitco.in account, we split it into 3 parts - the first part is used to cover user withdrawals, the second part is used to bankroll our MULTIPLY BTC game and the third part is used to expand our bitcoin mining operation (which is currently over 6 Petahashes or 0.18% of the total hashpower of the bitcoin network). We make a profit and assume the full risks of our businesses and in return for you saving your money with us, we give you a risk-free fixed rate of return.