I wouldn't trust backify.com or livedrive.com for the reason that they sell "unlimited space" for a flat fee. There is no such thing as unlimited.
The Backify guy(s) obviously never heard of this and tried to offer a free service of 512 GB. After all, LiveDrive service was for "unlimited" space. What's a couple hundred gigabytes anyway?
However, if you were to take p(x) as the pdf of people needing exactly X storage and take the standard assumption that the integral over it's domain (0->infinity) = 1, then you can work out the expected amount of storage required per customer (integral of xp(x) over it's domain). Now as long as the cost required to host that expected amount of storage is less then the amount they are being paid, well then, they still make money.
But what usually happens is that they take various 'measures' to cut off the right tail of the pdf to make it more profitable. That's dishonest.