Acronym to remember: POSSE = Publish On (your) Own Site, Syndicate Elsewhere (/Everywhere).  Question: is 1999.io an implementation of this idea?  

Answer: at this point, i don't know.  Twitter.com would seem like a natural syndication target, since 1999.io uses it for authentication... But the easiest way i have found thus far to push posts there is from Radio3.io (a River3 implementation hosted by developer Dave Winer, i gather).  This method takes more clicks than it should, IMO.  

Syndication to FaceBook.com via their Instant Articles API is purported to be built-in, according to this Reviewer's Guide just published, but i have thus far been unable to discover how to access this feature.

As to other social networks: There does not appear to be any provisioning for them in 1999.io... But developer Dave seems keen on the idea, judging by numerous posts to Scripting.com in recent months, including one in which he spoke specifically of connecting to Medium.com (which Evan Williams said in a recent podcast that he would be open to enabling, IIRC), so i'm inclined to believe we'll see developments along such lines coming soon.  

Add Wordpress.com to the mix (the biggest fish in the blog-hosting pond, if i'm not mistaken), let the world know about all these syndication powers, and the world will soon be "Blogging like it's 1999!"