Many years ago (maybe 2008), I wrote a GreaseMonkey script to synchronize my tweets to my Facebook status. This was way before Zapier/IFTTT could be used to do this, and before there was a Facebook API that could do this.

It worked by having a browser window open all the time to Facebook, and the user script installed in the browser. The script would periodically fetch my tweets from Twitter's RSS feed (I don't think same-origin policy was being enforced for XMLHttpRequest at that point), and if the latest entry was new, it would set the status's input field value to the new tweet, and submit the form.

Now, I work at a company that uses both Slack and Mattermost (Slack for internal communications and Mattermost for working with clients), and I'm thinking of making a newer version of this to keep my statuses between those two synced so that I don't have to install Slack/Mattermost apps to do it.