BSD Unix is one of the oldest still in existence, dating from 1977. As such, it is rock solid with decades of bug fixes, enhancements and tweaks. See https://en.wikipedia.org/wiki/Berkeley_Software_Distribution if you want historical information on this fantastically stable operating system.
In the early 1990's, multiple BSD descendants were made Open Source and manage many of the worlds most stable servers. See https://en.wikipedia.org/wiki/Comparison_of_BSD_operating_systems for more information.
We at Daily Data have historically moved towards the most stable platform that gives our clients the services they need. In the early 90's, we moved to Novell Netware, then to Linux when the limitations imposed by Netware became overwhelming. Now, with the emphasis on workstations rampant in the Linux community (and subsequent loss of stability), we hare moving “back” to real, standard Unix. We have decided FreeBSD best suits the needs of our clients with excellent security, efficiency and expansion.