Ask HN: What social proof works best in A/B tests?

Many landing pages have company logos or testimonials to provide social proof.

Has anyone had luck in A/B tests with other strategies, like visitor counters or popups (a la https://imgur.com/a/cbteZqe, fomo.com and provesrc.com)?

I feel that measures to show users the popularity of a site would be helpful, but perhaps not. I don't have enough scale right now to reliably compare tactics myself so am curious where I should focus my energy.