Yeah this is really annoying. When someone disconnects from the game does it still count? If not, this games ranking system is bust. Really hope it punishes this behaviour because many console games I've played with leaderboards are completely ruined because of it.
I'm pretty sure it does count. I've been lagged out of many games and have watched my rank drop significantly. I even checked my stats, and some of the matches I dropped from it gave me 2nd, 3rd, of 4th place based on the number of kills I had before dropping. I believe when you are dropped it just leaves your current score, and that is what is ranked at the end of the match.
But what about those games where they force the whole connection drop and everyone gets sent back to the menu? That may or may not be a different case then being 1 player that drops. Players, especially in 2v2 keep forcing this to happen because they are the 'host' or w,e.