The problem is centralized entities can take away access to data you generated at anytime. They can also ban you for whatever reason they want. So if your livelihood depends on these services, it can cause a huge disruption.
If Reddit/Twitter's data is stored on a blockchain, then anyone can build a client to access that data and Twitter would essentially have much less power.
The problem I see is that blockchains are too slow, and too expensive for mass products like Reddit and Twitter. Heck, I'm sure it's already a huge challenge for centralized databases to have as much data as Twitter/Reddit - let alone a decentralized one. Maybe at some point, we'll have a quantum computer breakthrough and blockchains performance is solved for use cases like this.