Passive social discovery happens when we casually observe information streams emanating from our social circles and pick up on something that strikes a chord. For example, I may see a friend of mine link to a blog post, or become a fan of a product on Facebook and as a result I have discovered something new

Active social discovery happens when information is actively requested, or “crowd-sourced.” For instance: I want to know the best place to get lunch in Santa Monica, so I post a question on Twitter asking my followers and solicit a response.