Google Trends monitors search terms, not traffic. The Google Trends graph is relative to the total search volume for the keyword Quora.

Web traffic is a very different thing. It's how many people actually visit a site. Here's an estimate of Quora's traffic: http://www.quantcast.com/Quora.com

You can see that while they've experienced a significant peak in traffic that has resulted in a recent decrease, they're doing very well.