We also see this phrase used to say that things seen on social media are not authentic, but are manipulated, such as people only posting their good news and not bad news, or people using photo manipulation software to change how they look.
I think currently most people are spending too much time on social media and paying too much attentions to the photo they are posting. I agree that people tends to posting their good news and photos and I believe it can lead to a skewed perception of reality, where it seems like everyone else is always having a great time, looking perfect, and living their best lives. But it's important for us to remember that the post is just a small piece of their lives and there's more behind the scene.