There are concerns that your publicly posted information would scooped up by bots that scrap up public information on the web. Or more specifically, be used by Meta to build a profile on you, which it already does even if you don't federate with Threads.
People who are concerned about this usually choose not to federate with Threads, but they also would need to block bots and Meta specifically to fully be protected.
Others don't share their concerns as much, or are more selective about what they post publicly. Some platforms allow you to post privately, for example, and unless you are communicating with someone on Threads, Threads would never see it even if you were federated with Threads.
If you decide to make it public. Or if you're on something that doesn't leave you any choice like Lemmy.
If you're on Hubzilla or (streams), and you've grokked it enough to use it accordingly, then you can actually post content in private to only selected users.
There are two common fallacies. One, the Fediverse is inherently private because it isn't corporate. Two, the Fediverse is inherently public because everything on Mastodon or Lemmy or whatever is the only Fediverse project you're familiar with is public.
If you're on Hubzilla or (streams), and you've grokked it enough to use it accordingly, then you can actually post content in private to only selected users.
Okay, but then Meta won’t be able to see it even if you federate with Threads (unless you share the content with Threads users), so I still don’t see your point.
Both Hubzilla and (streams), in practice the only Fediverse server apps that have a "public stream" and users other than the dev, can do a lot to keep content private.
But tell that to the Mastodon users who only know Mastodon and the Lemmy users who only know Lemmy, both of whom "know" that nothing in the Fediverse is private because nothing on Lemmy and effectively nothing on Mastodon is private.
Does it even matter if they know? The private content is supposed to be invisible to them anyway. The fact that they don't know it exists would make it more invisible.
Also, some platforms, like Mastodon, have actually adopted some privacy. For example, they added "followers only" posts that only their followers can see. If they are aware of Mastodon's "followers only" posts, then they already understand the basic concept of limited distribution.
Also, having the biggest instance\service is a power because whatever you do you need to make sure it is compatible, so you end up servicing them first and foremost, and they can pull some strings from their side to change things for everyone.
I don’t have a FB or Insta account… for a good reason. I don’t want one. Apart from Meta existing solely to maximise shareholder profit (which sits weirdly with the fediverse) there is its user base to consider. Whilst undoubtedly there are a lot of them, how many would bring actual quality as opposed to quantity of posts? Also, the Zuckdroid. Need I say more?
I think one of the issues might be if you’re subscribed to an active community of a few hundred people the fear is that community would mutate if suddenly a few thousand new accounts joined it and you could lose what you’d made.
Having said that, would I like a world where a ‘gram influencer stumbles in to a hexbear or .ml community and posts some vapid “sponsored content”? Yes. Yes I would. Would I like to see an evangelical turn up at Blahaj (sp?)? Yes. Very much so. Not for the drama - but to discover the contrast to their own personal truth. But, like you say, the tool-o-meter is already throwing out readings as it is, so everything would probably descend into name calling and bad faith arguments.