Quora is creating a giant collection of well-indexed, compelling bits of content in a format that Google finds delicious. They're already in the top 1000 sites and have been rising steadily. And they're doing it in a way that users are doing most of the work.
The obvious comparison is Wikipedia. Whether or not they'll get to that level I can't say, but it's not crazy to suggest that they will, and if they do they'll be worth a lot more than $500m. And even if they don't, they'll have a lot of users, a lot of pageviews, a smart staff, and a solid technology platform. Odds are good they'll find something to do.
I agree with the Wikipedia comparison. I'm seeing many of those Google searches that have Wikipedia pages as the first result now with Quora gaining ground fast.
There are two things that Quora is doing that aren't novel, but they are doing them really, really well:
1. Building an amazing topic taxonomy. A big hierarchy of topics that interest people.
2. PeopleRank. Who is an expert on which topic? Have they linked their Twitter/Facebook?
If that's what they're creating, they're not worth $500 million. Let's do the math: Let's say 5 million uniques a month, X 10 pageviews per user(that's asking a lot too) = 50 million pageviews a month. Let's say $10 CPM (that's asking a lot too).
That's $500,000 a month or $6 million a year at best. Great for a 1-2 person team. Not great for a startup asking for $500 million valuation. The pageview business model depends on huge huge mass. Not curated, premium Q&A. Hence why Yahoo Answers get a ton of traffic: because of a bunch of bullshit questions being asked every minute. Quora is the anti-thesis of that.
I think your math is a little off. When the Huffington Post was acquired (for $315m), their traffic was 15m daily page views. Assuming Alexa's not too far wrong, Quora's traffic is about 20% of the Huffington Post's at the time of acquisition, so figure ~90m pageviews/month already.
But really, they're just getting started. Quora has been open to the general public for less than two years. I expect their traffic will be much more of the kind Wikipedia gets than the Huffington Post does: long tail, evergreen content that gets a ton of free traffic from search engines. For that, and for the browsing that they so wisely encourage, you don't need lots of bullshit questions. You need lots of good ones that will turn up in search results, plus plenty of answers they find satisfying.
And there's plenty of room to grow. Wikipedia gets 100x the traffic Quora does now. I don't think Quora will ever be that big, but the $500m valuation isn't obviously insane.
If I'm manipulating the statistics in my head properly, even with a skewed sample, Alexa stats should converge on the truth as you go higher in the ranking, because the distribution of web site accesses is very skewed towards the top websites, which should have the effect of limiting the error as you get closer to #1. If Alexa says a site is well within the top 1000 (as opposed to just on the border, and Alexa shows them at 771), I'd accept that as strong evidence.
All free web usage data is terrible. (And probably all the paid stuff too; it's a hard problem.) But I trust Alexa for order-of-magnitude relative position for consumer web properties. That's mainly because I had access to the internal stats for a couple of sites like that for years and the Alexa stats weren't egregiously wrong along that axis.
The obvious comparison is Wikipedia. Whether or not they'll get to that level I can't say, but it's not crazy to suggest that they will, and if they do they'll be worth a lot more than $500m. And even if they don't, they'll have a lot of users, a lot of pageviews, a smart staff, and a solid technology platform. Odds are good they'll find something to do.