I do have enough knowledge to weigh in on the 'twitter is useless for selling books' thing but maybe I'll sit on it.

Ok I'll say something: the ones who say it's useless tend to be published by big five imprints and, importantly, they're YA writers. YA readers don't read small press or self-published titles and there's a correlation: this is to say, YA readers trend toward books that are--

--backed up by $$$ marketing, which includes but isn't limited to literally buying shelf space in B&N. For these writers, twitter isn't going to make enough difference. They're not after selling 10 or 50 copies; they 'need' to sell 5000 or 10,000.

YA is also more dependent on libraries, especially school libraries. Again, it's a genre that doesn't really do well in any circumstance but 'backed by $$$$$$' because that's the way YA readers pick books to read--via a lot of hype. And books backed by $$$$ need to sell a lot.

Small press and self-published writers, obviously, look at a very different scale and--save for larger small presses with enough $$$$$ for retail distribution--are not going to have titles in physical stores, and rarely in libraries.

So, basically, small press writers will notice when twitter moves copies; big five authors (especially YA ones) will not because the numbers don't make enough of a dent as far as they're concerned.

This also means that, in what I've observed, small presses and self-published authors do EXTREMELY poorly when they try to do YA. I could name the YA imprints of some larger independent presses. They're all gone now. Thrown out for years.