Okay, let me just say that by you being on this forum you have the advantage, you obviouslly know some PU stuff so you have an advantage. Now the other guy is probably an AFC, it is soo easy to post on someones Facebook all the time and women will like that to start with but nearly every woman I know gets guy's chatting her up on facebook all the time because they have no nuts.
My advise is keep playing it the way you already are. You are getting IOI's and your taking it further and further, if the subject is still bothering you on say the second date then bring up the guy, for all you no he may just be a work colleague that she know fancies her but she doesn't know how to let him down... It could be anything.
I think looking into qualification is the key, you'll want to get her qualifying herself more to you, by her telling you she likes you that means she is qualifying but just get her to jump through a couple other hoops too.
By your description though your not way off... don't worry too much about that other dude, if and when you go out with this girl in a relationship if thats where you want it to lead you'll find this cropping up a fair few times if your lady is particularly attractive.
Hope this helps
