[ganjianwei]

Growth matters because Quora raised a ton of venture capital and needs to eventually generate revenue to justify the high valuation, and there's a limit on revenue/user they expect to generate. Simplified version of web apps' revenue calculation:

Total revenue = revenue per user * no. of users

There's an upper bound on how much each user generates in revenue (either through ads, subscriptions), e.g. imagine paying > $200/yr to use Quora, unlikely that more than 100,000 people are willing to do that. So the fastest way to grow is through having more users. On top of pure revenue/user number, there's the network effects of having more users, as well as becoming a key part of cultural fabric e.g. most celebrities have Twitter accounts now.

[tokenadult]

imagine paying > $200/yr to use Quora, unlikely that more than 100,000 people are willing to do that

It's unlikely that more than ten persons are willing to pay $200 per year to use Quora in its current condition, and it's even less likely that those ten would stick around once everyone else left.

In other words, your example has even more force than you suggested.

AFTER EDIT: By the way, user droithomme nails it with his user's-eye-view description of Quora elsewhere in this thread.

http://news.ycombinator.com/item?id=4603653

AFTER FURTHER EDIT: I just did some specific searches on Google, site-restricted to Quora, which breaks through Quora's wall of obscurity and allows me to see current highly voted answers to questions on topics of known controversy in online discussion. The answers are crap. They don't even cite good sources, but are mostly anecdotes of lower quality than the typical anecdotes found on HN, and in some cases the anecdotal answers directly contradict what careful scientific research has found to be the general correct answer for the same question. But on many issues, Quora is just the opinionated leading the opinionated, and no one makes the effort to LOOK UP better, more reliable sources. I like the user culture here on Hacker News much better. I read whole books, for fun, after learning about those books on factual topics here on HN. I see no reason to put up with Quora's annoying user interface to get into a community with such poor quality control on the answers to the submitted questions.

[witoldc]

All of these online communities face the same problem:

As they get bigger, the quality of the membership becomes severely diluted. Eventually, the crud fills the pages, the 'good' members leave, and all that remains is the crud.

This is happening on Quora too. Look at up-voted answers on non-tech questions. All it takes is a bunch of nicely formatted paragraphs, lists, bolded text, and a bunch of nice pictures. Tons of posters are commenting on places they have never visited and things they know nothing about.