If I loosely define "constellation" here as a group of earth satellites where different ones are visible at different times, and you need all or most of them to get round-the-clock coverage, some examples might be: Iridium, Doves, Galileo, Glonass, OG2, QZSS, and GPS.
I'm wondering, were the Molniya's the first practical use of the constellation concept?
note: for this question geosynchronous is fine, but not geostationary. Millitary, commercial, and scientific constellations are all fine.