How did I survive this shock?
You got lucky.

Electrocution isn't an exact science. There are multiple things which make a difference:


The current which flows through a person depends on the resistance. That in turn depends on whether the skin is wet or dry, the area of skin in contact with the wire, and a lot of other biology things.
The resistance of the things completing the circuit - whether you're touching a metal radiator, or standing on a rubber doormat for example.  
The path of the current through the body matters too, it is currents through the heart that are dangerous, so getting a shock leg-to-leg is less risky than left arm to right leg, for example.
Release time. If you drop the wire quickly, there is less risk of damage.  The automatic response to let go of the painful thing might happen quicker or slower depending on all sorts of biological things.
Health.  Some people are just more susceptible to shocks than others. This might depend on build, body fat percentage, or just a pre-existing heart condition.


And of course voltage, source resistance and frequency of the source you're touching also matter.  All in all, US mains is in the range where it's not guaranteed safe, nor guaranteed to kill, so it comes down to the factors above, and a healthy dose of luck. 

Don't try it again.