“MSF” is the term for a group of slash fans who hop from the fandom for one show or movie (less often, book or comic/manga) to the fandom for the next. This usually happens when a BNF fic writer becomes interested in a ship in the new fandom. Quite often, the ship has an element of Any Two Guys/White Cock to it. The fans who follow the BNF will tend to write the same ship in the same way as that BNF.

::David Attenborough voice::

“And now, we have the privilege of watching the Migratory Slash Fandom. These magnificent creatures have finally exhausted the resources of the lush Plains of Marvel, and now as one, they move south in search of a rich source of their principal sustenance, Woobie and Whitecock. Very little is known about the way in which the Migratory Slash Fandom locates and selects its new territory … And now they're off. Never before has the spectacular sight of the Migratory Slash Fandom been caught on camera as they move off together in their thousands, clouds of dust rising, cries of “Squee! Squee!” echoing in the air, all the way to Teen Wolf…“
- source

Not to be confused with Migratory SJ Fandom, although the two subsets are not mutually exclusive.

A few discussions:

• Sep. 29, 2012: ”I've just realised exactly why I hate MSF!”
• Feb. 6, 2016: “What exactly are the elements of MSF fic that make you point the finger and shout MSF MSF MSF?”