Do you mean “selling” or “sharing”? Ya know, the links at the bottom of the page, saying: “Share this page”. I’ve never seen anyone literally selling Facebook, for instance; don’t even think you can. Do you also mean, having an ad on the side of the page, on Facebook? Or, do you mean attracting visitors to follow their feed(s)? Can you give examples of sites that do this? There are a bunch of web-developers on Fluther who might be interested in knowing.