I share studio space with two others. Cristian Rubén, a glamour/fashion photographer was showing me the power of instagram. He took a photo of a flat he had just painted black and posted it to his instagram feed with a bunch of hashtags. He said, "watch this." He then made me note the current number of followers on his account (4055). Within three hours he had over 5000 and the next day the count was over 8000.
So what, you say? It's nothing to click on the button that says FOLLOW. But remember this is network building. Each one of those people will see your posts come through their feed. You don't know who is watching.
The next day I started using Instagram a little more appropriately (with hashtags) and my followers went from 260 to over 1000 in a day. IN A DAY! *
Guess who will be posting to FB and Instagram on a regular basis now? ...
EDIT: *Sorry, this is an error as I was looking at a different account. However, I did see about a 5% increase in just one day. Thank you, Dan M, for pointing this error out.