Leviathan is pretty active around 6pm to 2-3am Eastern with various PF for various things. Though just like any other servers, there will be more PF of things people do most like Atmas or Leviathan EX farming or Coil 2 but that's the same anywhere you go. Leviathan has a big range of FCs to choose from based on your need: hardcore raiding, social or casual. At peak time, you'd probably find 15-20 PF up. You might want to ask for invitation into several endgame or social LS depending on what you like cause people make groups there as well.