Including a Fediverse ID (ex: ＠joebloe＠example·com ) in your WriteFreely (blog) post will cause that article to show up in that person's notifications.
(Well, assuming their Fediverse software has a way of showing notifications.)
...
So, you could use WriteFreely as a way of writing a letter to someone. (Just include their Fediverse ID.)
And you could CC people, too. (Just include their Fediverse IDs.)