Howdy is slang, it actually means "how do you do" (How are you?) It's simply the same as saying hello or how are you? Quite often when people say "how are you?" They don't really care, it's just another way of saying hello. If you're meeting a friend, you might ask. How are you? How have you been? How have you been doing? Whats up? Hows it going? *I'm not American, I would never say howdy. It's very American specific slang

You definitely need a response from an American! I would say it only if I wanted to imitate a cowboy.

People who say "howdy" would say it every time. It's just their personal form of greeting. As you are aware, it's regional. It's not used much in the Northeast, but if I heard it I wouldn't think twice about it. It wouldn't sound funny. I would just think "sounds like he's from Texas." It is a shortened form of "How do you do?" It's no different from any other shortened from. You might hear "How d' y' do?" "Howdja do?" "How are you doing?" "Howr ya dune?" I personally say "How are you?" or "How are things going with you?" or "How goes it?"

it is only used for the first meeting i think