The docs state:
"On systems with 1,000s of ZFS file systems, provision 1 GB of extra memory for every 10,000 mounted file systems including snapshots."
What does that mean - how do these people use ZFS? Are they using ZFS inside per-user mounted files? Or does it refer to really huge datacenters??
I always thought ZFS advocates for a single file-system, a giant one. Why do the docs talk about people mounting 1000s??
You are missing some of the basic concepts of ZFS. ZFS is meant to be one large pool of disks. After you have established the pool, you are suposed to put file systems on the disks of the pool. All of these file systems then benefit from the pool's underlying RAID layout and other IO characteristics (ZIL et al). Seeing 1000s of file systems reside on a pool is a possible scenario on larger systems.
A potential use-case for ZFS that would see thousands of filesystem mounts would be an organization that uses ZFS for NFS-mounted home directories. Imagine a large company or small university with zpools dedicated to home directory exports. Each user could have a dedicated ZFS filesystem with separate quotas and parameters. If rolling snapshots are implemented, it would be possible for users to see previous versions of their files by descending into the ~/.zfs/snapshot directory. Self-service restores!