Since it's a new feature, you can basically advertise it like most forums and wikis do with their new info. I don't see why the news announcement system is basically only used right now for people who are hacked or warning, why not make it a way to ensure that everyone sees a new feature of the site; say something like "Share a thread with your friends through social media now! Earn a trophy!*"
*You can add this or not, if there's a way to make a trophy for this it would be a real incentive like referrals. This, obviously, would entail a method of tracking how many times a user shares something through social media. You can make a set of bronze, silver, gold, etc trophies like exist right now through the like system (e.g. 'Newsman' Shared X threads with friends through social media).
I've never been one to get embarrassed in real life about my online habits so I actually share a few memes or interesting posts with them through Facebook messenger or even group chat functions by copying + pasting links.
I'm just spit ballin' here.