For all those in the dark – Instagram is all about posting pictures, tagging people with messages regarding the picture, and using hashtags to describe how you feel.
The reason why people find it cleaner then Facebook, and more acceptable is because there is only so much you can do on Instagram. It’s not like FB and G+ because it’s so much more limiting.
As for G+ – it’s practically now part of Gmail, so all those regular BY girls who have Gmail will most probably have G+. Most don’t use them and I’m sure some don’t even know it exists. And the pictures posted publically on those accounts are usually the profile pictures of their email accounts that Google automatically saves to their G+ profile.