A user only needs to create a transaction if they upload something. The 175k Facebook requests per second that they compare to is mostly just people looking at stuff, which wouldn't require a transaction.
Nope, that's a low end estimate of writes.
"Every 60 seconds on Facebook: 510,000 comments are posted, 293,000 statuses are updated, and 136,000 photos are uploaded."
https://zephoria.com/top-15-valuable-facebook-statistics/amp...
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