An “act of speaking one’s thoughts aloud when by oneself or regardless of any hearers,” is how the OED defines “soliloquy.”
Maybe the audience is listening. Maybe they are not.
Maybe they care. Or maybe they don’t.
Like characters in a play, we share our thoughts and observations with the world, or with no one, to move plot along; to situate ourselves — our motives, desires, and feelings — in the context of the drama unfolding around us.
Our soliloquies establish ourselves as characters.
Maybe posting to social media is really about trying to define our own agency as actors in a story we can’t control but need to tell — the story of our own lives.