There's definitely value to be created, but the catch is focusing. Someone will pay for the best data on their problem. Very few people are interested in buying 100 dumpsters full of random text: I can't see general services like Quora ever being worth much.
Metaweb is an example of the forced liquidation problem with knowledge services that take serious VC.
Freebase had no business model and Metaweb took $57 million in VC. Then Google took the public service, which had been built up by a large community, and effectively buried it.
As with most of the other cases, their only possible path that involved sustainability and long-term knowledge value, was to not take VC, stay lean, and either API their system for a fee (not a huge business), or run an ad model. Either way, their business case was small, and they took a lot of VC. The end result, another dead, formerly promising, knowledge service in an increasingly long list.
There's definitely value to be created, but the catch is focusing. Someone will pay for the best data on their problem. Very few people are interested in buying 100 dumpsters full of random text: I can't see general services like Quora ever being worth much.