I think a good social network should have the following features and priorities:
Promote real-life activities and suggest to go there with friends. There should also be free activities on this list, not just paid events. It should be human centered, so all activities should be listed based on what you like and what's good for you. You can choose yourself how much the algorithm promotes (mental) health, and how much it promotes what you like, for example.
Open People API
Open API for status-updates, so it can be integrated into other networks. Integrate other networks that have the Open People API too. This way you will be connected with all your actual friends, regardless of on which platform they're on.
To get on the platform you need to be verified using your ID, passport, or drivers licence. This way, only verified real people can get on the platform, with one account. Also, friends can verify that you're you. Also, it should be a possibility to only connect with people in a real-life setting. This makes sure you can't connect with someone you've never met.
Promote Small Communities
If your social network includes more than 150 people, you are going too sparse. The human mind is not made for handling so many connections. The human being originally lived in tribes with little intersection with other tribes. A tribal or community setting would be best to mimic this.
On most social networks you can share something with all your friends, there's no distinction with what kind of friends. I think it's much more natural to be able to share some things just with close friends and family (your inner circle), while sharing other things with your 'outer circle' too.
Freemium business model
Unlike Facebook and many other social networks where the business model is centered around screentime (the more screentime, the more they earn, using ads), a freemium business model would work much better.
Open Source Marketplace
It should be possible for other developers to create apps and plugins with your data. Because all apps are open source, experts will be able to verify that your data isn't sold or used in a bad way.
Facebook and many other social networks know everything about you, and sells/leaks this information to everyone except you, in order to serve better ads. A good social network doesn't sell any data, but shows you your personal statistics, to teach you more about yourself.
Looks after your (mental) health
You can activate plugins that collect information about your (mental) health so you can be informed when it's going wrong with you or one of your close friends.
Enforced by goverment
I think such a platform can only be possible if it's enforced by the government. There needs to be lobbying to make social networks more open so they can be integrated with one another. Let the people own their social graphs!