Learn English – “He” or “she”, “his” or “her” for an ambiguous name

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.

But is here a preposition and when a pronoun follows a preposition it is in the accusative case. If you substitute except for but, which more or less has the same meaning, you will see that it has to be her.