That's a very delicate matter, b/c as soon as someone percieves that you've hijacked them or their friends to deliver a marketing message on a social network, you have a worse problem than you did before in simply being ignored in a banner ad. Case in point, Facebook's Beacon program last year. Pushing messages aggressively into feeds also risks alienating users who might even be fans of that brand. Why not just be as clear as possible about what's advertising or marketing in social media? After all, Google found success with search after separating organic from paid results, making it easier for people to separate upaid content from ads.