Twitter is racing app.net to the grave. Google social is almost as forgotten as Apple’s flails.
Title, Link, Comment, Tag — all editable forever. RSS everywhere. No comments (no trolls), no images, no ads. Cash supported - $11 a year . No obvious string length limit. Common API and bookmarklet support. XML, JSON, and Netscape Bookmark export formats and API for programmatic transfer. Minimalistic mobile support because that's for apps.
Almost a perfect microblog foundation — save that it requires a unique link for each post. 
Privately held by a brilliant iconoclast (eccentric?) with atypical values. Maciej has a regular cash stream, seems uninterested in further growth, does no marketing, and his ongoing costs decrease as storage and processing costs fall. He is unlikely to sell or terminate prematurely. Pinboard’s longevity is largely bounded by the health of a male born in 1976 who enjoys travel, is probably a non-smoker, and knows the bus will be fine. Another 30 years seems achievable. Even Wordpress is unlikely to last that long.
Pinboard may become the world’s longest lived microblog platform.
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 I had to go incognito mode to find pricing. Turns out I paid when he had some kind of lifetime fee. A yearly fee is better.
 Pinboard has (editable) Notes which have Title, tags, and description (markdown formatted text). They are a handy way to create a text string with URL and RSS feed, but their native display omits the description portion and I don’t know of any app support (Pinner.app does not show Notes). I also don’t know if there’s API support for Notes or how export works. Notes are basically incomplete, but could be extended to create a complete (spartan) microblog framework. One could create a root “Note” and then, using Pinner, author posts as linked-lists of bookmark referencing prior bookmark … (hence unique url for each) …