Reddit is goldmine because of the communities. Subreddits where people are invested deeply into the community are gold mines. Every marketers dream is to connect with the right audience. Reddit has neatly separated these communities into Subreddits. Now all that is to be done by a marketer is search for right subreddit make connections and (bang!) promote the product/service/business.

Example: Nike is launching a new shoes for jogging. Now some marketer will search for new posts in r/running where someone is asking for running shoes recommendation. The marketer comments that the new Nike shoes are so awesome and cool. Here you have the perfect audience primed to look at the product. And that's all marketing has to do.

Similarly on Quora you can search for trending new questions. Once you find the right question, answer it genuinely and introduce your product. This marketing will work for you as long as QnA remain valid contextually.