The message urges people to repost if they don't want Instagram to make everything they've ever posted public.
Well, here's the thing about that. It's fake. A hoax. Not real.
The poorly written "legal" notice made the rounds on Facebook back in 2012 and was debunked, but not before duping people into reposting the message. And now it's back for round two, except someone boldly replaced "Facebook" with "Instagram"—literally in bold font.
Even though the notice is a hoax, when you opt in to use social media, you have to click "agree" to the Terms of Service. So unfortunately, you've already given away the rights to your data.