What is the best way to respond to students adding teachers to facebook?
Do as with any other person you don't want to have on Facebook: ignore the request or cast a silent no (pro tip: if you configure Facebook in pirate language, it becomes "begone, ye scurvy dog!").

Most people will get the idea: you don't want them there, and will not think further. If someone insists beyond reason, use Facebook's tools to report harassment (chances are you are not the first nor the last one). For anything in between (a message saying "would you like to be friends with me?"), a polite answer saying you don't feel it is appropriate should work. 

If you are asked in real life about it you have two options:


Be direct and say "I prefer not to disclose it" or "that is personal".
If you want a more diplomatic indirect way, claim you don't use it much and change topic.