People were supporting the likes of Prost and, more particularly, Senna when he was at McLaren. Deal with it, Kiki. 
My quote of "Probably 75% of the worlds F1 supporters support Ferrari" came from an English commentor in the 90s. I believe it would of come out of the great Murry Walker's mouth as well. At the time (in the 90s and yes it is a long 10 years when I can't remember which races he said it in) I actually would of watched all my races in Italian and Slovenian commentary, but of course in this age and age I download past races and have a lot of then in English which would of been how you watched them. I don't rememebr whcih races or races it was but it was in the 1990s, probably mid 1990s when Ferrari were not so good and Murry Walker said that. He is English and so the comment isn't biased and it was said when Ferrari were not the best, thus if it is true then Ferrari have always had the most fans. I would say it increased even more later in during the Schumacher era of dominance.
Murray Walker, as much as he is a likeable man, is prone to getting things wrong and gross exaggeration. Ferrari may have more fans than other teams, particularly from the casual, one-race-a-year fans, but hardly by as much as the epic proportions Walker is trying to make out. At any Grands Prix I or people whom I know have been to, there's been a good mixture of fans for many teams and drivers.

Ayrton Senna: WDC 1988,
1989, 1990, 1991
McLaren: WCC 1974, 1984, 1985, 1988, 1989, 1990, 1991, 1998,
1999, 2007McLaren: WDC 1974, 1976, 1984, 1985, 1986, 1988, 1989, 1990, 1991, 1998, 1999, 2008