I think it is just the amount of favor points you gather. I cannot really back that up though. My rank went to 1000ish after just 16 or so games. After that I would not know what other stuff gets into your rank. I think this xp is just favor points.
Then again the rank would be a function of the number of games played if that were true, and I think it is not quite that simple...
I don't know, good question!