It doesn't have to be a ride from a bar. They offer it through December so if you go to a xmas party and have 2 drinks and don't want to drive home, you can call specific cab companies and they'll drive you for free.
I think they do on certain days. I don't frequent bars tho so I don't know for sure.
We have one cab service in my town and it is VERY expensive.....New Years is a bonanza for them(and usually a 3-4 hour minimum wait time), I doubt they would ever offer a free ride....The main(and largest) social gathering spot does have a shuttle service that is pretty flexible with drop offs, especially on holidays