Well, a couple of options, anyway. Decentralized, so you can manage your own security. And, if you join someone else's, you don't know whether they've modified the open source code to collect more data than is generally done. And, since decentralized, not really connected worldwide? But possible ...
I say bring back Usenet ...
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Decentralized absolutely does not mean that the information is not shared, or that it is not connected worldwide.
The moment you share information with someone else, unless the system controls what can be done with the information (re-share, print, copy etc.), the decentralization has absolutely no meaning and no effect.
It's just a new name and a new way to eventually end up with exactly the same problems.
Then again, anything that takes people off of Facebook and any other mechanism owned by them, is a good thing in my eyes.
In some cases, in some departments, in some scenarios, you have to block access to Socials and avoid the risk.
If requirements are not that tighten, you can allow socials and use DLP tools to prevent copy/paste of documents and contents, implementing data segregation between corporate and personal contents.
If you want to use socials like a collaboration tools, FB and other providers offer an enterprise-ready platform for this, so you don't use the public one, but an ad-hoc version of it.
If you need socials for work related tasks, eg Linkedin or official Twitter, you could proxy access to social, so you do not disclose company credentials with anyone, they're just proxied using Domain Services.