Halliday & Matthiessen (1999: 509):
A biological system is a physical system with the added component of “life”; it is a living physical system. In comparable terms, a social system is a biological system with the added component of “value” (which explains the need for a synoptic approach, since value is something that is manifested in forms of structure). A semiotic system, then, is a social system with the added component of “meaning”. […] Semiotic systems are social systems where value has been further transformed into meaning.
Semiotic systems are the means by which social (and other) systems form. No semiosis, no social system. In the unfolding of the universe, physical systems preceded biological systems, and biological systems preceded social systems; but social systems didn't precede semiotic systems.