Using a social network, you could simulate the diffusion of information about a product from the starting point of an advertisement. So theoretically, you could highly optimize your ad campaign to target just the right people that will spread the word of your product the farthest.
Yep. Gladwell talks about the marketing aspect of some of these ideas in this book The Tipping Point. Actually, despite being a "pop science" book, there's a lot of good stuff in The Tipping Point, in terms of providing good starting points to start exploring. Reading his book was one of the things that got me interested in this field. From there, I started reading the stuff by Duncan Watts and Albert-László Barabási and then some of the more technical stuff. It turns out that network science underlies and unifies all sorts of stuff. It's really proving to be fascinating... well, to me, anyway.