The playerbase has dropped from the average of 3200-3600 unique keys per day (beginning of january) to about 2800-3100 per day. Personally, I was expecting more quits.
Reasons for less players:
Patch GTX
no more holidays / exams / work
no more steam sale
solar flares
people got a life and a girlfriend
(I made that one up)How many left for what reason, I have no idea. I'd guess it's 40% GTX, and the rest distributed on the others, who knows.
However, it doesn't drop any further by the looks of it, it seems to stay at status quo.
Either way, as long as there are servers with people in it, I am not worried. The point is I can have a good time playing the game whenever I want, be it in the night or in the morning, there are always some players. So It's fine