Instead of answering, I went through all the responses thus far and made a spreadsheet of the answers. (This is absolutely non-scientific and possibly inaccurate, plus there's no data as to how well these strategies worked, but hey, I was curious and thought maybe others might be interested too.)
I also have a list of "Other" answers that didn't fit neatly in these boxes or struck me in some way-- I'll link those next.
@ElisesWritings Specifically mentioned genre-specific newsletters which I thought was a smart detail to consider. @joyce mentioned not seeing a lot of boost from FB/Amazon ads, and that mostly word of mouth has been key. @pretensesoup specifically mentioned Instagram working well in regards to time spent, and also targeting hashtags. @sagetyrtle mentioned taking workshops for learning to get better at promotion which I've been working on too.