@Arkandel, I think the strict dictionary definition certainly applies in many cases (emphasis mine):
a narrow exclusive circle or group of persons; especially : one held together by common interests, views, or purposes
This is distinct from what I presume @Thenomain was referring to as "play groups", which would be a non-exclusive circle or group of persons.
Having a lot of cliques can be very detrimental to a game, since new players just throw up their hands and leave when they can't break into RP.
At the same time, I think cliques are a natural part of any social endeavor. And frankly, if I choose to log into a game just to play with Bob and Harry, who are my best buds from elseMU and we've got our own little plotline going on that doesn't readily open itself to strangers, that's my prerogative. It may be a DUMB prerogative because eventually we'll get bored playing with just each other or Bob will idle out and then our story will be up a creek, but it's still my prerogative.