I'd say promo sites are failing because they suffer from the same marketing saturation and the ensuing lack of effectiveness. They just can't suggest (never promise) the kind of results that could be achieved five years ago.
If you want to think outside the box, Facebook is probably your only option. It's the only ad format that lets you try different ideas, whereas AMS is obviously locked into a set formula.
For instance, I was thinking recently that to reach cozy mystery readers, I should target gardeners, or people who like cooking and baking. Yes, it's blatant and maybe even offensive stereotyping, but you can see the thinking ... you target a likely demographic that probably also enjoys reading (your genre) and yet has nothing to do with books. That way, your target focus can be tighter while staying away from the huge pool of FB ads targeting those readers directly. I'm sure people have done it before, and still do, but it was still a lightbulb moment for me.
Facebook ads can let you approach your target demographic from an entirely different angle. Mind you, I'm concerned that FB users in their entirety are a declining demographic due to age. That's going to be a problem in the not-too-distant future.