I don't think those are the "ads" people here are talking about. As far as I know Instagram doesn't make any money off of personal endorsements like that.

They offer business accounts: https://business.instagram.com/

Which looks to have at least some useful tools: https://blogs.constantcontact.com/instagram-business-account...

Now I'm not saying every Instagram model getting endorsed for posting about some company's fitness gear has a business account, but I'm also sure that number isn't zero. There's a gray area where if you're making enough money off that type of thing, or are close enough to get there with the right nudge, that it probably makes sense to buy into it even though you're not necessarily running a business.

Essentially, you're paying into being (or becoming) a professional celebrity. My guess is the number of users in that niche isn't trivial.