In order to enable periodic snapshots you need
to add these lines to your /etc/periodic.conf

hourly_output="root"
hourly_show_success="NO"
hourly_show_info="YES"
hourly_show_badconfig="NO"
hourly_zfs_snapshot_enable="YES"
hourly_zfs_snapshot_pools="tank"
hourly_zfs_snapshot_keep=6
daily_zfs_snapshot_enable="YES"
daily_zfs_snapshot_pools="tank"
daily_zfs_snapshot_keep=7
weekly_zfs_snapshot_enable="YES"
weekly_zfs_snapshot_pools="tank"
weekly_zfs_snapshot_keep=5
monthly_zfs_snapshot_enable="YES"
monthly_zfs_snapshot_pools="tank"
monthly_zfs_snapshot_keep=2

To get hourly snapshots you also need to add
something like this to /etc/crontab:

2       *       *       *       *       root    periodic hourly