Can you be into the pairing without being into the fandom? That's something I'll have to think about ...

This is getting into a slightly different area, but the very extensive tagging on AO3 (we don't see it much in the MFU fandom except in the crossovers) has brought out a slew of readers who read for kinks as much as fandoms.

Another interesting side issue which doesn't really affect MFU is writing one pairing in a fandom rather than another because it's more popular and brings in more readers.