I travel a lot and have done multiple study abroads on different continents when I was in college. I find this to be a gift and a curse of Facebook. Its great when they are genuinely interesting people that you share lots of interest with. But its awful when you have a good night or even a good week with the person and then you friend them on Facebook only to find out later you don't have much in common with them. It really dampers your mood when you look back at the old pictures and you're having a good time with someone you deleted because of all their irrelevant wall spam and complaints. Some relationships are much better when they are fleeting and you only have the good memories.
Well, friending someone on Facebook is starting a relationship with that person. Just like I wouldn't call someone the next day if I didn't want to start a relationship with them, I wouldn't friend them either.
Friending them just to realize you have little in common is akin to dating someone only to realize you have little in common.
