I'm definitely assuming (a). I think we have different user stereotypes in mind. I think that lots of people spend little to no time hearing about, or talking about, applications they aren't using already.
These are the kinds of people that go to mint.com not because they've heard about it, and are excited about it, but because they typed "track my finances" into Google and it was the first result.
Well I don't disagree that in one sense you're making your life harder for yourself by adopting a platform that has a smaller install base...
But I do want to challenge the idea that you're necessarily making the right decision by trying to appeal to the widest possible install base. It might just be that this is more harmful than is commonly accepted.
These are the kinds of people that go to mint.com not because they've heard about it, and are excited about it, but because they typed "track my finances" into Google and it was the first result.