How can I flirt via Facebook?

[Sammy]

Don't be too random about it. Just find a topic that is a good reason for you to message him and continue the conversation from there. If he is interested in you, he will reply you and keep the conversation going, but if not he most likely won't and you don't have to worry about it.
If you do keep the conversation going, don't go directly to talking about stuff that will make the conversation very difficult. All you have to do is, be subtle, say things about him that you really like and finally meet up with him or something. After all, it's over the chat, nothing too horrible will happen.