My response would most likely be to point out the misunderstanding and try to better identify myself to the person. "This is "Mr. X from Light City. Are you sure you've got the right person? I went to King's Jr. High..."
I see no real trouble in keeping the post up if most of my friends will recognize the mistake and know that I am not arranged to be married to anyone. I would not engage with the confusing details of post.
As its your best friend that has posted this and it says "you have an arranged marriage with one of my best friends," perhaps you could say "Wow, I'm arranged to be married to me, I'm wondering if the Best Man and Father of the bride can use the same speech, would like to know what you think."
Thinking about this from a teenage perspective (the students I teach), I would imagine one of my students would respond with something like "It's awesome that you are best friends with your mom!"... implying that you are arranged to be married to his mother and soon to be his father.
This scenario is exactly why I dislike Facebook. Things on the site (postings and "likes") are so utterly random that it throws me off.
As for how would I reply? I think that I would post something like the following:
If we are related because of this marriage, I am getting a divorce. I don't want to be related to anyone who has nothing better to do than arrange fake marriages.
I would probably respond with some kind of question about how this person knows things before I do. "wow are you psychic? I didn't know I was arranged to be married. MAybe I should ask around before I go on that date on Friday." or something to that effect.
I like post 3's comment about how my spouse would react to find out I had an arranged marriage to someone else. Perhaps I woud respond with some humorous confirmation of the so-called marriage, finding some method of communicating to all who read the first message and my response that the first method is not accurate or truthful.
Beyond that, I would probably seriously question whether or not I wanted to continue with this person as my "best friend." I don't think I would be able to appreciate that type of humor in my friends.
I'm not sure how I would respond, other than to ask why I am arranged to marry the person. Perhaps I would point out how disconcerting my wife would be to find out I was arranged to marry someone.
I don't live in a place where anyone has arranged marriages, so I think maybe I don't understand the real dynamics of this situation. For me, I think I would go with trying to take it lightly. I'd say something like "no, it means I'll be your daddy." But I really don't think I probably understand this as well as I should.