I know. Poor critters. Not that it was remotely "worldwide", they didn't even get to Canada or Mexico, it was strictly nationwide (in the Lower States).
But one look at their opening acts during 1976/77 tells you how big they were, albeit as a strictly US-American phenomenon: Aerosmith, Lynyrd Skynyrd, Wet Willie, Marshall Tucker, BÖC, Rory Gallagher, J. Geils Band, Bob Seger, The Outlaws, Ted Nugent, Foghat, Foreigner, Nitty Gritty Dirt Band, Pure Prairie League, REO Speedwagon, Styx, Edgar & Johnny Winter, The Band, Montrose, Point Blank, Head East, Elvin Bishop, The Dictators, Santana, Rush, Nils Lofgren, Sea Level, Burton Cummings, Muddy Waters, Climax Blues Band, Piper, Blackfoot, Atlanta Rhythm Section and Starz.