Instead of fighting, I would suggest the opposite embrace it head on. Push and pull content from social media and the internet but make your forum the hub.
On my site I have been creating half a dozen bot accounts. Their job is basically to post contextualised hyperlink in threads coming from either RSS feeds or key Twitter accounts. With a bit of experimenting with a few rules you can automatically pull the best content /most relivent content on the internet.
If you liken it to a store. Facebook and Twitter are like a jumble sale. Besides old ladies jabbing you with elbows, it’s takes a bit of knowledge to find good stuff and spot the nuggets. Now apply that to a discussion say about US politics. The downside of these platforms is there are rabbit holes. From reinforcement bias to attention seekers, to those that wear tinfoil hats. Finding the best content takes time and experience.
Forums need to fill gap where people want their ideas and thoughts to be challenged (they want debate), they want to have minimum posting standards, they don’t want to engage with crazies.
If people can use your site to find good content elsewhere for me that is a good thing. People come to your site to find the best content. While there they are more likely to engage with other like minded people also seeking content they like or challenges their thinking.
Sorry went a bit philosophical there 
Key thing is though it is very very easy to use Zapier with Discourse (free 1000 actions a month). If you can bring content to your users which is more tailored and better your user base will grow.
You should also consider pushing content to Facebook and Twitter. For example every new thread automatically send out a link promoting it.