Marrying an Australian citizen is one of the ways you can migrate and start a new life Down Under. It is not rare for backpackers to find their better half while traveling the Outback. On the other hand, there are a lot of people willing to marry a citizen only for the sake of becoming a citizen themselves. This is a common issue to every single immigration authority, not only Australia.
Not anyone can join the group on facebook.com, so potential candidates will undoubtedly go through a screening process, before they can have their shot.