For a guy to add a hot chick to his Facebook account and then message her to and fro seems to have an opioid effect - the interaction pushes some pleasure reward-button in his psyche and he slips into fantasizing all manner of possible scenarios that have zero basis in reality.
He would be wiser to regard her as he would a babe in an interactive porn-site: sure she might messge back with cute emoticons but he'd be a fool to think that she gives a fuck about him any more than the hordes of other guys drooling over her. She craves the attention and/or cash - nothing more.
Hot chicks on facebook can have 1000s of 'friends' and they receive dozens of 'friend-requests' daily and with their morning coffee they sift through scores of indulging and flattering messages from lonely men.
Unless she's met you in person or you've managed to stand out in some exceptional manner you're just another faceless and meaningless male buoying her withered self-esteem with time and energy that will never be recompensed.
Apparently being afforded access to her private life by being allowed to view carefully hand-picked pictures of her whilst having a pop-up box appear in which you can write emotionless soundbytes to each other is a trap for a male.
Social media is an expression of a female's psyche - it does very little for a male.
The hard reality of the situation is that a man might think he's building upon a connection because she 'likes' his posts and writes witty comments and sends him messages but all of this is merely entertainment value for her.
Inevitably, one day you'll wake to check your friend's list and she'll be absent: perhaps you've been 'unfriended'; perhaps you've been 'blocked'. Perhaps she's deactivated her account.
Either way, all of your typing away on a colorful screen to a cute avatar was wasted time and energy: in 99.99% likelihood you'll never meet her in person, you'll never gaze directly into her eyes, and she will never suck your dick!