Might Delta Chat be adapted to become a social network with just minor modifications as an alternative to Facebook.
As a social network it has the benefits that all your contacts are already on it, and, you have an existing history with them in your inbox/email server.
The usual problem with new social networks is encouraging friends away from existing ones. None of your friends & family want to leave because all their friends and family are in it, etc etc Catch 22.
Most social networking consists of sharing, and commenting on, photos and videos.
It wouldn’t take much to make photo sharing/commenting a key feature and most people’s email server/inbox already contains a repository of shared photos and documents so you would not be starting with a blank feed which is what puts people off often.