I feel like I good advertising campaign could do wonders. Reddit’s userbase was beginning to decline around a year ago (if I’m remembering correctly) and so they had this massive marketing campaign with a focus on how there’s a community for everything and, then, they had a massive increase in users.
That being said, their results are not the best. I would probably recommend MetaGer.
I know Mojeek have their own database of pages that are not taken from Google or Bing or anywhere else.
Regardless of the difference between declination and softening (keep in mind it’s difficult to show when people stop using a platform as most don’t delete their account), the mass advertising campaign of theirs definitely worked incredibly well