if you are using zfs that is where your memory goes. By design zfs caches memory. It is called the arc cache. It will release memory based on system needs but is going to take as much as there is avaliable for its cache. It is recommended not to modify this and let zfs manage it for you, however you can adjust it by modifying your /etc/system file to minimize the amount of memory that zfs can reserve for its arc. If you want to see how much zfs is using go to this site and get arcstat.pl and run it on your system
Monitoring ZFS Statistic - Roman Ivanov
the output will look like so:
# ./arcstat.pl
Time read miss miss% dmis dm% pmis pm% mmis mm% arcsz c
14:20:55 343M 36M 10 34M 10 2M 10 1M 0 10G 10G
14:20:56 11 0 0 0 0 0 0 0 0 10G 10G
14:20:57 3 0 0 0 0 0 0 0 0 10G 10G
14:20:58 6 0 0 0 0 0 0 0 0 10G 10G
14:20:59 26 0 0 0 0 0 0 0 0 10G 10G
14:21:00 6 0 0 0 0 0 0 0 0 10G 10G
This shows me that zfs on my system is cacheing 10g of physical memory. I would be willing to bet you will find your 8g there.