The exception to that rule is Stack Exchange, because they have a business model that is unique to the space and impossible to replicate for a site like Quora (Genius, Answers.com, wikiHow, et al.).

Quora has to allow low quality content on their service in order to keep the volume up, to drive traffic & clicks, to drive ad potential, to avoid the dreaded down rounds and eventual drift toward forced sale. There's only so much legitimate high quality content for a site like Quora and it's nowhere near enough to validate a $2 billion valuation (much less higher).

Consider for a moment that Yelp - which is a real business in a highly monetizable segment, that is also profitable and will hit an annual billion dollars in sales soon - is worth $3 billion. So if you get a $2 billion valuation as Quora, where are you going from there? It's obvious.

Genius is facing the same exact fundamental problem that Quora is. Take a lot of money from VCs, get a big valuation, find it impossible to live up to it. Turns out normal people don't want to annotate everything and could mostly care less unless it's a more narrow passion segment (music).

There are only two paths for knowledge services. Stay small and very lean, aggressively limit costs, and use an ad model - that's wikiHow. Or go the Wikipedia route. Anything involving VCs will end in disaster and or forced sales. Knowledge services properly have to think very, very long-term (if they're actually trying to fulfill a knowledge mission and aren't just traffic fronts), they need a decade outlook or more. VCs think short-term, they look at ~5-10 year type exit outcomes. High quality, long-lived knowledge services are fundamentally opposed to a focus on exits in any manner, as they have a higher calling than looking for an exit for a VC - and any deviation from that must inherently destroy the community.