It sounds so perfect: you put a "share this" button in a banner ad and suddenly even stodgy, old display media becomes social media. Pay for the impressions and get the viral effect as a bonus. It's the future of advertising! There are just two catches to this: it's been done before, and it's not the future of anything. It's a bell, a whistle, or maybe a bell and a whistle if executed well. But it's not going to fundamentally change advertising, social media, or anything else.