It didn't go from $0 to $1. It was always "$0 to most, $1 for those that really didn't care about paying because they knew that anything else would have much higher real costs"

Hell, If WhatsApp ever got to say "Facebook wants to buy us, but we are also considering changing to a pay-what-you-want model to avoid selling out", I'd easily give them $10/year without batting an eye.