Steve33 To do this, we use our RSS feed and post on our Facebook page with #hashtags. This means that a lot of people come to our Flarum.
We do something similar. We use RSS to feed all possible platforms. This brings some users to us who stay. Of course, this requires good content ;-) We have only been here for two years. 2K members, 4.6K discussions, 65.5K posts