The fact that they're processing a lot of data is in large part because they need to for advertising.
Still, your understanding of blockchain tech is misleading here. Ethereum is public key registry at best and is not and should not be used to store or process data.
In a blockchain world you can still have service providers, but the user is the one with the power, not the service provider. Users are free to switch service providers as they see fit because their identity and data isn't tied to a single company.