Historically, I have used Google hangouts with success but am open to other ideas.
Well, Modern World, I doubt we will stay in America all the time.
PC's can come from any background that fits a modern origin. Medieval wizards are right out.
As far as whether or not PC'S know each other, it's up to the PC'S. It certainly makes sense considering paranormal investigations on this level are probably fairly uncommon.
Stay away from Cthulhuan Knowledge, although occult, parapsychology and similar skills are probably okay. Stay away from actual powers (within reason, an occultist having a meager spell or two I'm okay with, having the mutant ability to detect ghosts or goats I will probably frown upon).
As I said above, mystical, psi, or similar low level 'real world' powers I am okay with.
I wouldn't want to run with less than two or more than five or so.
You should always ask if I drink the blood of new players (rarely), if I'm from this planet (I think so) and whether it's possible to curry my favor as a GM (no but those who get dramatically or even comedically into character have been known to gain bonuses to rolls from time to time.)