It must be substantial, as sites like yelp are actually unusable on mobile web, as they constantly try to install the app.
It seems like it's better to lose a customer than to have them on web.
Most of the claims companies like Reddit make about security/anti-spam benefits is simply not true. Once you require user to create an account/login it makes no real difference. Everything you need for the frontend can be done with JS, HTML & CSS, and the apps themselves on ios/android are nearly-always just that.
As for losing customers, I need to use Reddit to make Google effective - and Reddit knows this. They have a massive amount of power to force mobile users to use an app if they wish to. Given it comes with massive revenue-potential, why wouldn't they?
Push notifications increase DAU/MAU by multiples.
Those are the killer features for social media.
I'd also assume that the walled garden of the app store cuts down on a lot of bots/other shenanigans going on.