I was not home when the process server came to the door. My downstairs neighbor let the server in and the server left the summons on my stairs. Is this being properly served?

What does it matter? You have the summons, are you going to claim you don't? Trying to claim improper service is just a delaying tactic. Its one thing to claim it if a default judgment was entered because you were unaware of the suit. But you are aware of it now so deal with it.

Improper will depend on where you live, often posting it on the door or in this case on the stairs you had to walk up, knowing you lived there would be good service in many places