There is this plugin for Twitter: https://getkirby.com/plugins/hashandsalt/twit
There used to be one for Instagram, but it stopped working when Instagram changed their API. Check out this thread Future of Instagram Plugin
As regards pushing content from Kirby to Twitter, I’m not sure if the Twit plugin already handles that.
But I guess for your purposes an integrated solution would probably make more sense. Maybe look to integrate some third party service?