Which one is correct and why:
It was the first time a girl asked me to add her on Facebook
It was the first time a girl asked me to add her to Facebook
Adding someone on Facebook is generally accepted as a shorter way of saying “While on Facebook, I added someone to my friends list. The statement is meant to express what site you were on (visiting) when you added the new friend to your friend’s list. Furthermore, when you “add someone on” a site, it’s understood that you are adding the person “to your friend’s list”, which is why that part of the statement is not said explicitly.
Adding someone to Facebook implies you are creating a new account for her on the site.