I don't even see any ads on their front page, which is the most prominent source of income for most faucets.
It's very likely that they're mining in the background and I would certainly not discount that as a possibility.
besides the very high withdrawal which you can only reach if you have tons of referrals under you, I stopped doing this after I found that very high threshold because I'm not good in referring.
Collecting lots of gems is easy, but the thing is their value decreases as your balance grows.
I managed to reach the highest level and got to 69% without taking surveys or referring a single person. So I can get hundreds of gems daily, but the amount of rewards I get decreases as I progress, and despite that I was willing to continue playing their game to the end, but now that I know they're abusing my machine. I'm not willing to do that anymore.
My point is, you don't need to do any referring to get the benefit of that which is gaining lots of gems, but even if you do have thousands of gems, you probably won't reach the threshold in months, if ever.
The fact that it's so easy to build up your balance, and there are mechanisms that essentially try to suck you into the endless cycle of slaving away for them (both your computing power and your labour), along with a high withdrawal threshold should be a sign enough that they have absolutely no intentions of paying anyone out.
What's the point of using them then even if they did not mine in secrecy?