I post a picture of me drinking a Coke to Instagram. I expect to share this with my friends, and maybe the people who follow me on Twitter. I know it's publicly available, but the chances of anyone beyond my circle of friends seeing it are slim to none. But now, Instagram notices that I posted this picture and sells it to Coke. Now, when Coke starts buying ads on Instagram, my photo shows on total stranger's news streams saying something like "Hey Enjoy a Coke, like this guy here!"
So, what's wrong with that? Well, for one thing, I never agreed to be in an ad campaign for Coke. Maybe I don't even like Coke. Maybe MY caption was something like "Ugh, out of Pepsi".
For another thing, generally, when you appear in an advertisement for a product, you get paid. Your likeness in the context of a commercial ad campaign has value, and when a company says "Yeah, I'm just gonna go ahead and take that and not pay you anything" the appropriate response is "hell no".