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Can I send messages independently from the stories I post on my website / RSS feed?

Currently that is not possible. We are focusing on what we’re best at: getting you maximum followers and making sure most people come back to your website after you published a new post.
If you need to inform your followers about something else, you can download your follower’s emails and send them a separate email with your standard email application. Or, you can incorporate your message into one of your regular posts.