I guess it's only normal to expect the prize pot to go down over time as Bitcoin gets more expensive and more players sign up every day if it's the case of wager + free rolls taken.
Yeah, I believe when bitcoin goes up that makes people spend less because they are spending more dollars if they spend the same bitcoins, however I also believe that in the long run when bitcoin goes up there are more people interested in bitcoin and by logic interested in freebitcoin as well, so that means when bitcoin goes up you could see some sharp drops time to time in the wagered numbers but over time that means the overall wager numbers will get higher and higher when dust settles.
I feel like whenever bitcoin gets a ton of attention from the mainstream world we end up with a lot more new people and those new people do not want to spend thousands of dollars right away to see what it is all about, so they do end up here getting some free satoshis and doing something with it.