Online matches are going to be the slowest way to earn FM since it is the only one that is endlessly repeatable. You get a lot more from the one-time FM purses from Story and Survival, as well as the character level-ups. Next month's update that brings new modes will likely also have similar FM opportunities, including the daily challenges.
I'm getting scared that the daily challenges might not be daily, but instead one time only like story mode.
http://www.eventhubs.com/news/2016/...iscovered-and-it-looks-there-are-nearly-7000/
They look static and per character. Hope I'm wrong though and it rotates per day.