If you are determined to use Facebook, I'd agree with what tobyhede said: the way forward is through creating a community buzz around your product. I log into Facebook pretty much every day (my group of friends got very keen on it at Uni, so most people I know use it!) ... but I've probably clicked on about two adverts in my whole time using it.
I have, however, joined a lot of groups (some serious, some silly, some "fan" ones). So I'd suggest creating a fan page and/or group for the product, or even a Facebook application that would give a "taster" of the note-taking software. (I'm no fan of applications myself, but lots of people are.)
You might find some of Seth Godin's writings useful, particularly "Unleashing the Idea Virus" (
Unleashing The IdeaVirus) and "Flipping the Funnel" (
Seth's Blog: Flipping the Funnel--new ebook)
Good luck!
Ali