Undoubtedly a massive proportion of commuting fans... however through the 90's I have to admit their away support was massively local and pretty top-notch (i can't really comment now as it's over 10 years since we played them).
However (whilst on an anti-Liverpool tirade!) Liverpool's support is just as cosmopolitan (spend an hour in Crewe railway station on the morning of a Liverpool home game and then tell me it's any different to Man Utd's support...