What is interesting is that your poll shows few people use social sites for this. This result merits further investigation since either you have a very odd sample answering your poll or hype is not equal to reality in the case of Instagram. Scope for a future article?
Do I share photos? Yes? Do I use any "social" site for this - no - precisely because there is insufficient control already, let alone if Instagram get away with this. "Social networking" sites are founded on the premise that more and more people will expose more and more information about themselves - madness!
For sensible photo sharing you can use....
... private folders in any one of a range of cloud storage services, including iCloud if you live in the Applesphere
... your local photo printing site (eg Photobox) where you can control specific folders
all of these limit the audience and preserve your rights over the pictures AND enable you to permanently remove the pics when you wish.
Of course, you cannot control what recipients of your pics do, but by choosing how you share and with whom you can have as much control as will ever be possible.
Why do so many use Instagram and the like? Laziness (can't be bothered to think it through and find an alternative), letting themselves be driven by "fashion", plain old showing off and a hundred other reasons - none of which make it prudent or wise!