It never ceases to amaze me how much lip service people pay to the need for privacy on social networks that are fundamentally based on the premise of broadcasting your private life to a fairly large audience. If you REALLY don't want that photo of the sunset that you took to be seen by the wrong person, why post it online in the first place?
1) Twitter is inherently much more public than Instagram. 2) "to a fairly large audience" is your personal assumption. Not all people use social media that way. 3) Photo of a sunset? That sounds like your being obtuse. Obviously many photos are much more personal than a sunset. 4) To be seen "by the wrong person"? Again, seems obtuse to me. The larger concern is the photo being permanently stored by the wrong corporation(s), to the eternal detriment of user privacy. 5) You are assuming that the user him/herself posts all photos of a given user. That is obviously not the case. Your photos, or photos picturing you, can be posted by others as well.
Suffice it to say, many users who do enjoy social media nevertheless do not want Facebook using everything they post for the next 25 years for marketing purposes. They don't want that content sold to other corporations, either. And there are many other possible legitimate privacy concerns.
It all depends on which "circle" or "bubbles" we live in.
In my mind, your arguments are flawed: my friends shared everything. My friends tweet their instagram food pictures. My friends posted their instagram sunset pictures on Facebook (seriously, I'm not joking).
Many of my friends and their friends enjoy social media and use Facebook heavily almost for everything including selling dresses, cupcakes, foods, services, shoes, lingeries, etc.
I'm not done yet: some of my female friends do have conversation about lingeries that they were tagged to on Facebook in Facebook.
Based on my "bubble", your "bubble" is the minority: a very small group of people who seem to be overly concerned with nothing.
It's probably cultural.
(1) that doesn't respond to his argument.
(2) no they didn't share everything. Presumably they shared what they wanted to.
And here's the problem with Facebook, explicitly: If you upload something to facebook by mistake and it's something you didn't want to share, Facebook winks and says "ok we deleted it" but this has been shown false too many times for users to trust that they actually have control over their own content.