From personal experiences, i follow a lot of big names in the niche i am interested in, people who have upwards of 100K real followers (usually they are 500-2m range). Very often they give shout outs, mostly to people who give them free shit, and more often than not i will click the link and checkout the other person's profile. I am sure many other do this too. However wether i follow them or not depends on what kind of content they have and how much it interests me.
I also feel that with Instagram people are much more willing to follow a lot more people than they would do on Twitter or FB. Instagram is only pictures with no external links, so a user can spend an hour looking at pics on his or her cellphone without being distracted by an outbound link.. With FB and Twit there's text-pictoral-video content all vying for eyeballs and if you follow too many others, your timeline can get very cluttered. No such problems in Instagram i feel. Just my two cents as a user.