I dunno about its changing popularity, but like Expert Sexchange before it, I actively avoid Quora. It was the first site I added to my Google Personal Blocklist, and the reason I installed the plugin.
Their unsubscribe form is (or was last I saw it in 2015) a nastier piece of work than I could imagine, even if told: Come up with the most annoying unsubscribe form you can. Make it really hard to unsubscribe from everything...make'em work for it.
Quora, I hope, will continue to lose to much better, much more ethical, Q&A sites. The Stack Exchange sites seem like the market leaders, as they probably should be. They do nearly everything right, and they do it without being smarmy. Sites that believe they have a right to my attention, and are willing to cheat to get it, really ought to be shunned in polite company. I'm not sure how they've managed to maintain a patina of legitimacy after all these years of being no-good, shiftless, internet hucksters. We, as a community, usually shun the hell out of spammers...and yet, when Quora (and LinkedIn, for another example of a spammer getting a pass) do it, most folks just shrug as though it's no big deal. Does a certain level of economic success lend credibility even when behaving in ways that deserve no credibility?
Not that I'm grumpy about it, or anything.