I doubt about Freebitco.in investment actually, they are starting from faucet gambling site and now offering an investment opportunity, I understand that the site got a good reputation, but I still not really trust it
What exactly don't you trust? Let's talk about it publicly!
Current biggest risk would not be your site but Bitfinex, their Wells Fargo troubles could make the value of a bitcoin drop significantly.
So you own a mining farm? Customers probably would like to see pictures of shelves full of mining equipment, or some hashrate statistics. Current or former data. I am not sure how those are verifyable as legit. On payouts they are proveable using a blockchain explorer.