Either the users download the whole social platform in their hard drive or they have to use a third party. It's that simple.
You said it exactly right, but this is a good thing, not a bad thing. I think you missed the point.
The user can choose how they use the platform.
The backend is the blockchain that allows a decentralized message system to function without third parties. The front end is whatever you want it to be. You can run a full node and verify all messages yourself or you can use someone else's website that does it all for you.
There is incentive for people to create websites or platforms that read the blockchain because they can then make money on ads, subscriptions etc.
There is incentive for people to use the chain for the obvious reasons that decentralized blockchains bring.