Instagram isn't improving Facebook's bottom line, but a growing number of entrepreneurs and marketers are figuring out how to use the photo-sharing service to bring in revenues of their own. Some offer services to Instagram's 100 million users; others use the network to share promotional images and links to products. "It's classic marketing. You see an ad on a billboard, or on a bus as it goes by, on TV and now, in an Instagram post. It sticks," says Mobile Media Lab co-founder Brian DiFeo.