If a girl writes on a guys fb wall?

[Monster]

Your question is legitimate, but please, I cringe whenever i see someone take facebook as just a harmless social media forum and not for the unimaginable force that will affect their life forever. Remember please, that anything you write on facebook, even the most harmless "Hello" will follow you for the rest of your life, your future schools will go over your Facebook page with a fine tooth comb to see if you are worthy of them, the Government will document every single thing you say in case they want to use it in the future against you, your employers will also go over everything on facebook for your entire life to see if you have ever lied, done something embarassing, etc......so be careful....oh so carefull, what you say.
And to anyone who thinks I'm paranoid....crawl out from under that rock and start reading.
And to finally answer your question, saying "what's up, how have you been" just let's him know that YOU know he exists and you like him enough to take time to look at his wall. But he certainly would'nt think anything romantic was up, unless he was "needy" and takes all female attention that way.