'Meet' is the ACTION. It is the moment when you say 'Hello' or shake hands with a person for the first time. It only takes a moment to meet someone. This is a dynamic verb, because it describes an action or event. 'Know' is the RESULT of meeting someone. Once you have met someone for the first time or been introduced, from that moment onwards, you know this person. This lasts a long time. 'Know' is a stative verb, because it describes a state or situation. Event ---> We met a long time ago. Result ---> We have known each other for a long time. Event ---> John and Anna met in 1998. Result ---> John and Anna have known each other since 1998. Note that we use the past simple for the first sentence, because it refers to an event in past time, but we use the present perfect for the second sentence because it describes a situation which began in the past and has continued into the present. Your three sentences with 'meet' are fine. I hope that helps.

The difference is pretty much the same as the one between "conocer" and "encontrar" in Spanish, though "conocer" can mean both "know" and "get to know" in English. You can say: – We've known each other for a long time. – We met a long time ago. – We got to know each other a long time ago. Your last examples are all fine.