I may be (read: am) biased, but I don't think there is (currently) any finer system in the world for technical, more-or-less-science-based Q&A than Stack Exchange.
So the open question is, how many technical, mostly-science based topics exist that work on the Stack Exchange Q&A engine -- and how large can they become? Many have been tried, some are working; see the statistics at http://stackexchange.com/sites?view=list#traffic
At the very least, Stack Exchange produced Stack Overflow, which is already absolutely enormous, growing like crazy, and well on its way to becoming the Wikipedia of programming. We can't really say that about Quora for any topic, can we? Will Stack Exchange ever recreate that level of success on any other topic? I don't know. I suppose you could plot growth curves of the various sub-sites and estimate how long it'd take for them to reach the size of Stack Overflow.
So the open question is, how many technical, mostly-science based topics exist that work on the Stack Exchange Q&A engine -- and how large can they become? Many have been tried, some are working; see the statistics at http://stackexchange.com/sites?view=list#traffic
At the very least, Stack Exchange produced Stack Overflow, which is already absolutely enormous, growing like crazy, and well on its way to becoming the Wikipedia of programming. We can't really say that about Quora for any topic, can we? Will Stack Exchange ever recreate that level of success on any other topic? I don't know. I suppose you could plot growth curves of the various sub-sites and estimate how long it'd take for them to reach the size of Stack Overflow.