Always publish on your own platform then syndicate it across all the relevant social media providers to get exposure. It's a huge pain, especially now that most platforms are closed and don't provide automation (APIs, RSS), but it's really the most flexible solution. It also makes syndicating to new providers a lot less painful since you'll have some standard raw form (e.g. Markdown) which you can create a manual or…
Standard reminder that Markdown, while pleasant to author in, is perhaps one of the least standard formats ever (every implementation does things their own way; see Babelmark 2) and not a great archive format, largely as a result. (Markdown files don’t have version or flavor information embedded) I hear org-mode is a good alternative. In the mean time, consider storing a permanent .html version upon publication of ma…
The major point of using markdown, or why I use it, is that it doesn't require a working implementation to be understood. I can open a markdown file in ed and still understand it. From what I've seen of RestructuredText and Org-mode, if i'm not using the implementation, it becomes a mess.
(I also always recommend to put anything that isn't widely implemented in Markdown in HTML instead since that will be reasonably parsable for the next couple decades.)