My son is 16 and has had a FB account for a couple of years. The rules that I put in place were simple - full access if I wanted it. When he first opened the account, I checked it often to ensure that he was practicing on-line safety; not cyber-bullying or being cyber-bullied; not posted derogatory items; "friending" known persons, not random strangers; etc. As he progressed and grew his social media connections, and I realized that he was being uber-responsible, I checked less and less. Now, two years later, I never check.
Same for text messaging - I used to check it constantly. Now, he is the one who reads me his conversations if he thinks it will amuse me.
Same for on-line gaming - I have "met" some of the boys from across the country that he regularly plays with via chat on the gaming system and when he Skyped with them. Turns out their Moms have done the same thing. Now we all talk about meeting up in real time one day.
It is all about developing trust and boundaries. My son and I have been navigating these waters for awhile now. I have learned that keeping the lines of communication open while allowing for privacy and growing independence is a double edged sword to the Mom in me. I want to keep him "safe", but have to let him go, in order for him to grow into the man that he will be.
None of us know our teens. Heck, our teens don't know themselves. They are all moody and hard to be around - hormones, societal influences, increased self awareness - all collide at this age. Our kids are trying to figure out who they are and where they want their place to be in the world.
Your decision now is what to do with the FB account. It is time for open dialogue about it, social media in general, and what types of boundaries you expect her to keep when engaging in those activities. Before you speak with her, you need have a mental picture of your expectations and you need to objectively look at those expectations and decide if they are realistic or not. Take your feelings out of the equation.
Teenagers naturally push their parents away - it is part of growing up and gaining independence. It sucks for us parents, but it is the right thing for them. But as naturally as they push us away they also continue to need us to help guide and encourage them. But you have to begin doing this through conversations, not ultimatums.
In the grand scheme of things I don't think a FB account for a 14 year old is a bad thing. If she didn't want you to know about it, she would not have switched computers with you. You admit that you are not sure if you had "the FB conversation" with her. But, now that the ship has sailed, you need to haul up anchor and sail on with her.