The activitypub standard isn’t limited to tweets. There are Reddit-like and YouTube-like implementations.[0]
The privacy would depend on the server rules. You could have a long/medium form blog that is federated using activitypub.
But there’s a significant technology toll to pay to get these up and running in my experience. It’s still not like signing up for Twitter or facebook. Which probably makes for better communities once you’ve managed to make it inside.