Content is mostly irrelevant, metadata is way more interesting. In other words they do not care what you say, but who you say it to, when you say it, from where, at what time, and so on.

This is not true. They munge through the posts. In fact, that is one big reason they can target people so well.

Not very, but getting them to outright do so would be an excellent step forward for privacy.

I wouldn't be so sure of that.