As soon as BF4 goes beta, or rather BF3.1, BF3 will start to die. As soon as BF4 (3.1) goes live BF3 stops. Oh, not literally, but in almost every way. It's the way of these games. There will still be servers, no doubt, but even today it's hard to find some game modes. After each expansion pack there was a surge then it died down to a core set of maps and play modes. For example, try finding a tank superiority map -- there's only 1 server that's up. Used to be tons. Try air domination: just a handful.
Later this year it will die almost entirely.