“I knew people were going to ask what the bandage was in the photo, and I knew I didn’t want to say,” she said. Luckily, the situation was resolved with a quick text to the new friend, who promptly removed the post before too many people saw it. Still, Ms. Blanco said, that moment changed the way she thinks about interacting with people online and off.

“It made me more aware of and sensitive to what other people might not want to share,” she said.

In “Discipline and Punish,” the French philosopher Michel Foucault theorized that the mere suggestion of surveillance is enough to alter our behavior, as we internalize expectations and monitor ourselves in an effort to conform to them. This was, to Foucault, ultimately more threatening to an individual’s personal freedom than actually being locked up behind bars.

So what does the threat of being photographed midbite at a friend’s birthday party, or cutting loose on the dance floor after one too many, do to the texture and quality of those moments, particularly when the image may end up being seen by several thousand followers?

I’ll go out on a limb here: Foucault probably wouldn’t have been a fan of social media.

“People are absolutely modifying their behavior in what I consider to be the most frightening ways. They are, first of all, mentally surveilling themselves, mentally keeping an eye on what they’re doing,” Dr. Nippert-Eng said. “We need to start thinking about the social costs of losing our privacy. In a democracy you always want people to feel empowered to say and do things that are unpopular but important. Today, many people feel like they’re being constantly judged.”

And what about when it’s for someone’s business? “When I did a makeup trial for my wedding, I went to one artist who posted an unflattering before-and-after picture of me on her Instagram without my permission,” Avery Carpenter Forrey, 31, said.

Ms. Carpenter eventually decided to shrug it off. But experience like that touch on questions such as: Does using a service equal consent to be used as marketing material for that service? Did agreeing to have her photo taken mean it could be used — filtered and edited — in any way the poster wished?