CrashPlan is "unlimited" but it's not a storage service, it's a backup service. From what I understand, you have to use their client software, and that software will use RAM proportional to the total size of the data you're storing. In practice this puts a hard limit on the amount of data which is proportional to the amount of RAM you have.
So heavy users are not only capped, they're also subsidized by the light users.
So about 170mb/TB if this scales linearly for some reason.
Some users were apparently bragging about having 1PB+ on ACD. That would mean the RAM usage would actually be a useful upper bound, as I doubt many of these guys have 170GB of RAM in their media server PCs.
So heavy users are not only capped, they're also subsidized by the light users.