How do those who serve subpoenas find the one they want to subpoena? Do they look with their social security numbers? Do they talk to family members? How do they find the person?
All of the above (except for social security numbers ... info attached to those is generally hard to access even for the police).
I did this for a short time many years ago and mostly we went on the address indicated on the subpoena and any other addresses provided to us by the involved party or parties. If we got paid enough, we would do a little hunting and searching and this was in the day before computers were everywhere. Now it would be much easier to find some people.