Ookla, you've also got librarians, teachers, and other gatekeepers in growing numbers starting to recognize the value of manga, so I think that's probably part of it, though I'm sure most of it is word of mouth through the readers themselves. That's an advantage that books written for the below-18 crowd have, though--if a book is well-reviewed, librarians and booksellers pay more attention, and are therefore more likely to read it themselves and handsell it. I'm sure for manga it's different than other children's books, but that's the buzz I'm hearing about manga from librarians.
I think reviews make a bit of a difference in the adult market in a similar way, but not nearly to the magnitude that they do in the children's/YA market, because in that market, the parents (usually controlling the money, up to a certain age at least) listen to the recommendations of the other gatekeepers a little more.
Advertising usually makes no impact whatsoever--reviews and word of mouth, radio and other interviews, and especially TV publicity like Oprah's picks or an author appearance on the Today Show are more likely to be influential than advertising. I think the reason is that with books, people want to know more about the story--either the story within the book, or the story of the author.
Since few books get put on NPR and Oprah, though, most authors have to settle for trying to get the word out via blogs, publisher-sponsored PR, appearances, signings, readings, etc.