@brutus5000 Not necessarily, while it's only anecdotal my own personal experience was that I did the UEF original supcom campaign in co-op when starting off playing with someone I knew (who was new to the game), primarily for the reason noted above - it helps teach the game, and trying to start on the first FAF mission would have turned them off the game due to the complexity, assumed knowledge, and difficulty.
If people are new to supcom then they're more likely to lose interest if thrown in the deep end than if introduced gradually; by contrast if they're already familiar with supcom they've likely already done the original campaign and so would be more likely to look for the more challenging missions (since it's reasonable to assume they'd know about both the original supcom and the FAF missions and go scrolling down for the FAF ones, whereas someone new to FA altogether wouldn't).
That said, based purely on what games I've seen hosted, the FAF missions are far more popular than original supcom missions (although how much of that is down to the order theyre presented in I dont know)