Maybe I'm getting some semantic distinction wrong. Each user's thread is essentially a cryptographically linked list. It differs from Bitcoin for example in that there's no change of ownership, and so no forking or input/output transactions, it's just a linear list of events. There are no gas fees or tokens, since there's no need to pay for anything, each peer voluntarily gossips information.

The description you cited is also probably more for marketing purposes than technical explication. No one wants to use a social media platform built on ETH, the experience is just horrible.

Yup, that's just a Merkle tree.

In a blockchain, blocks occur at regularly timed intervals, and are distributed globally, which is all enforced by some distributed authority mechanism, like Proof of Work. Rumor has it that Satoshi was originally going to call the data structure a "Timechain" but wanted to emphasize the cryptographic integrity feature more than the time keeping feature.