The free Apple app Instagram has announced that it has 15 million users. What’s the big deal? For starters, it’s really a social network. You can “follow” other Instagram users after you register, and can leave comments on your favorite photos, just like with Facebook. What’s neat about the app is that you can apply filters to your photos right on your iPhone before you share them, to make you look like a quasi amateur photographer. Read more from Ragan, including brand-building tips: click here.