I put this in the Heavy Rail discussion, too, but since Chicago's ridership increase is quite healthy I thought I'd post it here, too. The only two places you can say grew better (rail) ridership than Chicago are New York and LA, and LA has been adding heavy rail lines plus started from a very low base. New York's population growth helped a lot, I'm sure. Probably the reduction in overall crime and crime perception helped a lot, too. If Chicago's crime continues to improve, that should be a bonus for ridership here, too.
Graphic created by me, from APTA numbers