Dalton is right that advertisements that are indistinguishable from content are the best kind of advertisements, but only if they are displayed in a context in which the ad-viewer (read: user) wants to see that.
Google has this explicitly tied to a query (and implicitly with previous queries and increasingly with mobile device information, for Google Now). If a user sees an ad on Google it is almost certainly comparable or better than organic results with respect to RELEVANCE TO THE USER, which is what matters. Users express their intent, and Google only shows ads alongside the content that are directly related and aiming to solve the same problem or achieve the same goal.
On Facebook, what is the parallel? I'm on Facebook so that I can keep in touch and communicate with my friends. My intent is to see stories about my friends about their social interactions, parties, etc. I want to find something to do and see what they've been up to. I want to see the cool new product they bought in action and pictures from the amazing vacation they took - what I don't want to see is that they bought /something/ from Amazon or that they're planning a trip I'm not invited to. Facebook is about sharing your life, your past, your "timeline", not your purchases and plans.