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.
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).
(510000 + 293000 + 136000) / 60 is about 15.5k, so slightly less than the estimate in this article, although you are right that I was overstating the amount of traffic that is read only.
Yes, but that's not counting things like post likes, FB Messenger, check-ins, account creations, profile edits, logging and statistics, etc etc. I wouldn't be surprised if they're in the millions of writes per second.
In a recent submission [0] they give some stats about their abuse system and at ~47:35 they mention this system gets 1M requests per seconds, which doesn't have to precisely coincide with write volume but suggests it's in the right ballpark.
Decentralized apps are structured very differently than centralized apps. It doesn't really make sense to talk about about logging in that way for a decentralized app.
