I believe you, I just have not heard of many people making a killing of it.
How would you sell to your Instagram followers? (if you had instagram)
It would be incorporated into an overall digital marketing strategy. You don't use instagram for the ecomm (selling) part, but more for the message and awareness part. It's a very cost effective way to reach your base.
Look at it this way - what would it cost a company to get a piece of information to directly to a market segment that uses it products or follows the industry? They'd first have to ID who they wanted to target, then pay for various means to deliver the message. It would cost a bundle. With instagram/twitter, etc, it costs nothing.
This is why companies offer discounts to their products when traffic is driven to them from other sites - it's because it's cheaper for the company to be contacted by an interested party through someone else's medium than it is for them to charge full price and identify and market to the consumer themselves......
