Remember that time when you posted something on Facebook, and all the people who liked your page raved about it and clicked right through to your website? Yeah, that memory is fading pretty fast. Or how about that time when you had how ever many MySpace friends and you thought you had totally made it? Tumbleweeds, right.
There is no guarantee that any external platform will stay the same forever. In fact, most of the evidence is to the contrary, and no matter how stable Instagram might seem right now (breathe!), there really are no guarantees that it will go from strength to strength, or even that it will be around in 5 years. The only thing that is guaranteed is that if you actively build your email list, you will always have a way of communicating with your clients.