Esp. OK as an acknowledgement.
Used in Instant Messages and Social Gaming chat. KK is easier to enter (55 55) than OK (666 55), or hit k twice.
Common use has caused KK to become a terminating interjection similar to Goodbye, especially when used alone.
Rich: I95 is blocked into town.
Audrey: kk I wont go that way
Guild: Hey everyone, raid invites in 10 minutes. Get on your main.
Altpally: kk, back after quick bio.