Solid login across the entire internet

I have been looking at Solid and other related projects for a while(like SAFE network and Blockstack), as I am working on a project in which I hope to build upon one of these frameworks.

One big issue I keep running into is scalability and I have an open-ended question that I’d love feedback on from people who are involved in building Solid itself.
On your website, you talk about how you can eventually log into sites with your Solid account and your account(pod) will hold data from these sites. For smaller developers who build products on the platform, that’s great, but how do you see this scaling into something that can work on large sites, like a Facebook for example? Is this a case of developers making the decision to change their code and write on Solid or is this something that can be worked around?