Gab.com but people hate on it saying its far right wing whatever and they are banned from App store.
There is Mastodon. Its a neat idea, basically you set up your own "Twitter" . Literally. You can buy webhosting and install it, and it will be like your very own Twitter that people can join and use. You can join other people's "Twitters" , they call them "instances" and if you have an account with 1 instance you can read and message people registered on the other instances/servers and follow them. Some of them are specialized like : tech, sports...etc.
here are some examples:
https://mstdn.io/public ,
https://masthead.social/public ,
https://linuxrocks.online/public
Apps to use: Tusky for Android seem popular, and Tootle for iOS.
As with all open sourced projects, the problem of Mastodon it lacks the final "polish" on software and lack of users.