FreeBSD is borrowing memory for disk caching. The kernel is grabbing all the memory and it doles it out as processes need it.
You can look at the output of the vmstat command and the 'fre' column -- I believe that shows the same value as 'free -m' shows on Linux. The command 'vmstat -m' shows detailed memory usage.
You can also look at the output of 'top' at the Swap line and if you're not using swap memory you should be fine.