One of the pains of the Fediverse (where the original incident occurred, and ones quite similar to it do frequently), is that the posting-scope options are:

- Public: everyone[1] has access to the toot, and it's listed on public timelines.

- Unlisted: everyone has access to the toot, but it is not listed on public timelines, and hashtags if used aren't ordinarily discoverable.[2]

- Followers only (FO): only accounts which follow your own can view the toot.

- DM: only accounts named in the toot, and yourself, can view the toot. Oh, and admins of the originating and any recipient instances as well.

The problem is that the followers-only scope is set for each toot within a thread. I'll often face the question of responding to another person's FO toot, and doing so myself FO, which means that my set of followers can see what I'm saying and potentially glean context, or as a DM. I'll often reply DM so that only the recipient sees my response, particularly if there's any sensitive information involve.

On platforms which have a post-and-thread mechanism, such as here on HN, but moreso sites such as the late Google+, Diaspora*, and from my understanding, Facebook, responses are already scoped by the top-level parent's visibility. (On HN that's pretty boring: posts are either visible globally or not at all, it's more interesting elsewhere.) Microblogging systems such as Birdsite and the Fediverse ... don't have that, so context collapse within FO threads is far more likely.

Google+ also had the notion of "Circles" ... clunky, but usable, which were defined by the poster and which could be the target audience of a given post, and (in time) Communities, which were shared groups, public or private, open-admission or moderator-approved, to which third parties could subscribe themselves. (Similar to subreddits, generally.) Both did provide ways of defining scope and context of discussions, with some reasonable bounds on privacy.

There's also the challenge that multiple participants within such a thread may not be, and in all likelihood are not mutually visible to one another, so that the view of all participants but the thread parent author is likely to be partial, with fragments of the discussion open, and broken threadlets appearing and disappearing as followers and non-followers comment.

Then there's the whole notion of setting my privacy scope by who has chosen to follow me, which is to say, an action I've virtually no control over. The Fediverse has an additional setting which can somewhat manage this by requiring specific authorisation of follow requests, but that itself is clunky and cumbersome. It seems to me to be far more sensible to define a group either by explicitly joining one, or by defining one myself, and setting the posting scope to that group of my explicit choice or definition.

The upshot for all of this remains largely the same: using the Fediverse for highly-personal communications is probably a poor choice. But, see my earlier reply on thread,[3] people's sense of "public" vs. "private" online seems to be quite poorly tuned and not easily addressed by any means, technical or otherwise.

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Notes:

1. Excluding various block mechanisms which are ... complex in their own way.

2. The exclusion of hashtag visibility from Unlisted toots strikes me as among the more profound design/architecture failures of Mastodon. It's quite often that I don't necessarily want public-timeline exposure but would like hashtags used to be visible to those with a specific interest in them. Recent changes (v. 10.4) with some really useful search improvements (as in, it exists at all, but also specific filters and criteria) make this much less painful.

3. <https://news.ycombinator.com/item?id=37733680>