This... is the internet. It looks like you know how to do it. Yeah the best way is to point out how random you interacting with them is and then say why you added them. And see how they take it. And go from there.
I still don't see the point; so you get to know that a girl who you find attractive talks to you from somewhere in the world? Or does she lives near you I am confused.
It seems like you could either
a) find attractive people near you where their attractiveness is actually relevant to your possibly sexual relationship or
b) interact with people on the internet for things other than how they look b/c... is that... important?
Also it should be duely noted that porn exists.