If it's what you say, just on face book, then it's not sexual, not even slightly, you're son has misinterpreted it, I'm sorry. They'd even post it about same gender folks or even their pets or their parents.
It's simply what they say on social media. Often a pisstake, othertimes it's just a compliment, the odds are overwhelmingly she does not think he is hot. In fact if he asked her out or tried it she'd be horrified.
Sorry, he's not really offay with comments on social media and you've screamingly over reacted. You both have. Not sure how you'll break it to him gently that his luck really isn't in. And to say she's not backing off as she said hi is just crazy talk.
I have a nineteen year old daughter, I can see her face book page and all her friends, and have done for years, comments like this are just having a laugh.
I've just got a bad feeling uour son is going to get a massive embarassement and piss take coming his way if he tries it on with this girl. Or tell people she fancies him. He's making a big mistake. Sorry. Based on this there is nothing to assume she fancies him, in fact the opposite, if she did she wouldn't have wrote it. Tell him to stop being a prat.