I am not exactly sure how Quora does it but I would guess that they use something like Google Analytics. It looks at what percentage of your visitors come back each day, every other day, each week, etc. It's a good metric system that is crucial for building a meaningful web product, and it's almost always positive. If someone comes back to your site regularly each week, or even better each day, it means you're becoming a regular and important part of their life. The problem with any other single metric system is they can be the result of positive or negatives changes to your product. You need to have a near-wholistic picture and Google Analytics can help with that.
Tools like Klout and Hubspot are good with the measurement of engagement on a brands different channels (website, facebook, twitter, gplus...) too.
Let's chat some more and get down to the heart of this problem. Hopefully I can help you resolve any internal discomfort around the matter.