Most of those writes don't need to be serialized transactions. Other p2p data structures like CRDTs and torrents are sufficient for posting comments and uploading photos; see the decentralized social network Patchwork[1] (based on scuttlebutt[2]).

The blockchain itself will only be needed for certain types of transactions, for instance those around identity (e.g. name registration), and reputation/spam prevention (forum moderators).

1. https://github.com/ssbc/patchwork 2. https://github.com/dominictarr/scuttlebutt