Ask HN: Are Twitter, LinkedIn and Quora ad platforms any good?

Hi HN, Google and Facebook are basically a duopoly on digital advertising. But I am considering if there is an opportunity in what I consider to be the "long tail" of digital ads: Twitter, Quora, LinkedIn. I am the solo founder of a bootstrapping business, with a niche product (help managers with their one-on-one meetings). I don't have the budget to go for the best keywords on Facebook and Google (both in terms of ads and SEO). But instead of going for the long tail of the keywords on the major platforms, I was planning to go for the major keywords on the long tail of the platforms. Does that make any sense? Does anyone have first-hand experience with advertising on Twitter, LinkedIn or Quora? I would love to know how it went.