It currently has zero revenues
Its operating expenses are currently funded by over $160 million in angel and venture capital investments, including a large investment from founder Adam D'Angelo (former CTO of Facebook.)
Investors were willing to fund that money (which values Quora at over $1 billion as of mid-2015) because they understand exactly how Quora will eventually make money.
The sole mission of Quora (the company) is to generate a database of great answers to questions.
Those answers, for many reasons (including timelessness, quality, content, structure, source, vetting, etc) will often be at or near the top of any web search on the question.
Despite Quora having a relatively small number of subscribers, the fact that its answers are so highly ranked means it already has over 80 million unique monthly visitors and is the #140 most visited website in the world...with most visitors not being pre-existing subscribers.
Quora will eventually sell contextual advertising, which will run alongside appropriate content and therefore be valuable.
