I'm of the opinion that online communities that focus around a specific topic or group of topics cannot remain focused with growth of the community over time, and they cannot be sustainable without a certain amount of growth. Either way, they will fail eventually (the former due to dilution, the latter due to alternatives) and this is perhaps the reason why no online community can last for relatively significant amounts of time.
Let's assume the first scenario, where an online community has a standard success growth curve (albeit at a very healthy growth rate). Examples of this include Hacker News or Reddit. Let us also assume that the community exists to discuss news, innovation and ideas around topic x. They would have a growth curve as seen below:
The line labeled 1 in the graph represents the amount of topic x orientated users or members in the community. Line 2 represents the average intelligence of the community (or it could also represent the intelligence output of the group as a whole). The general rule of thumb is that as membership grows, discussion is diluted (thanks to off-topic or banal posts) and thus intelligence decreases. In the graph above, we see a community grow and start off with a fairly niche, targeted membership that goes through its growth phases, eventually hitting event C which is the critical mass required for the community to stay active. As it grows, because it has reached critical mass, a more varied crowd will be attracted and the community will eventually be given some form of media coverage - event B - (which will almost certainly be preceded by blog coverage). It will then eventually reach event A, which is success. The definition of success is debatable, but we can assume success either way.
What happens now is that because the community now plays host to a wide variety of members, discussion about the original topic will be a lot more diluted, and will most likely detract the hardcore, original members. They might move away from the community, diluting topic x discussion even further, until eventually everyone interested in the subject leaves, leaving behind a community that will pivot to an off-topic focus, or a community that is doomed to dissolve altogether. This is what I think will eventually happen to Quora and Baraza.
Alternatively, a community's growth can evolve with a selective or exclusive growth, as seen below: Lines 1 and 2 represent the same variables as in the first graph. What happens here is that the community's growth follows a similar starting pattern, except much slower, and plateaus at a certain point. Because people that aren't directly interested in the core topic don't join, the discussion and thus the 'intelligence output' doesn't get diluted. We can consider Stackoverflow as one of the examples of this type of community. Then event A happens, which is an introduction to the market of an alternative, that is gaining more traction (because the alternative is following graph 1's curve), and a large share of users leave due to herd mentality. (I kind of screwed up line 2 in the graph - it should have started dipping a bit more at this point). After enough time, and after enough people have left, the online community would have lost their required critical mass that's needed to keep the community healthy, and it suffers the same fate as the community in scenario 1.