The problem is that social media reduces us to the way we present ourselves. While that certainly is part of who we are, it's not the whole story.
One of the most popular maxims of ancient Greek philosophers was "know thyself", and the reason they considered it important is that it turns out to be a lot harder than it sounds. You think you know yourself, but chances people who spend a lot of time in close physical proximity to you understand you in ways you don't.
But online your identity is mediated by how you present yourself. This is not only inevitably somewhat dishonest (in ways that may be more obvious to others than to yourself), even when you are trying to be honest you at best are presenting who you think you are.