I personally try and grab new members that share similar interests. For example, I have a forum now (not run by Flarum) that focuses on Volkswagen owners here in Australia and around the world. It has not officially been launched yet but I intend to do this within the next week. I posted something about it on a PC and gamers page on Facebook and there has been significant interest already just from people in that group.
I guess taking advantage of off topic areas in such groups could be something to look into seeing as Facebook is quite a large platform with millions of users and you aren't breaking any rules so long as you adhere to the ones posted in those specialised groups. And best of all it's free.