Facebook's photo sharing app Instagram will start showing ads in the timelines of U.S. users this week. They'll look just like photos from your friends but with a little label that says "sponsored" on it. There are pitfalls for this sort of thing -- once advertisers arrive, some loyal users may depart. But ads have always been part of the expected evolution of Instagram. In fact, the app's already got advertising -- for guns. Will Oremus at Slate recently wrote about this, and tells Marketplace Tech Host Ben Johnson how gun ads got into your feed.