What's appropriate way of using Facebook as first step when found old friends?

Facebook is great to reconnect both of new and old friends.

However suppose you found very old friend you knew long ago.

What's first thing appropriate way without causing shock or discomfort or unnecessary fears to another human being?

What's first step should I do to person?

Send messengers first to introduce yourself? Or send friend request? Or do nothing until I met them by accident somewhere first?

Ya know doesn't want to violate Facebook rules or doing unethical to people.