Glide lends itself more to your first use case, and Glide have made no secret of the fact that their primary target market is what they refer to as “dark apps” - apps for private organisations/businesses.
That doesn’t mean that Glide can’t be used for your second use case. It can - up to a point - but if you’re looking to build the next Facebook, Tiktok or Twitter - then Glide is not the right tool.
I’d recommend having a browse through the Template Store to get a feel for what type of apps are being built with Glide.
Also, this interview with Glide’s CEO is well worth watching.
To answer you question about data privacy, the answer is generally yes. As the app developer you do have access to (most) user data. To learn more about how Glide manages user data and security, refer here.