>[1] No, PLATO was never nearly so widespread
Plato often gets overlooked, but it was highly influential. A well written book on it is: "The Friendly Orange Glow: The Untold Story of the PLATO System and the Dawn of Cyberculture"
The wikipedia page has some overview info on it:
https://en.wikipedia.org/wiki/PLATO_(computer_system)
In terms of usage, there aren't many stats, but I saw this: "The CERL PLATO system logged 10 million hours of use between September, 1978 and May, 1985 (a period for which the most complete statistics are available).