Woz is fairly well known as a philanthropist, and has been since the early 80s. I'd guess that he just hasn't drawn the same level of press attention that the Microsoft billionaires have. I'm also quite sure that he has nowhere near as much cash as them, despite how well Apple are doing now, they've had much harder times than MS.

I've no idea about Jobs though. His personal life has always been very secretive, and I would imagine any involvement with charity would be just as secretive.