Why does he log off of fb when I'm on?

[Bora]

I know that some people do not want to speak online with the girl, cause they want to see the girl in real, not chat with her, where you cannot see her eyes, and where you have all the time you need to plan what you're gonna say, or have a bad experiences over chatting with people, maybe because they end up being awkward or weird, and is afraid you're gonna change your opinion of him based on chat.
In your case, where things are uncertain, this case is completely viable, since he can surely screw up things over the internet.
Meet him in person and talk.
What matters is what he is in real, not in facebook, remember that!
Do not think this is because he is avoiding you, just doesen't wanna make things worse and is looking for opportunity, or thinking?
There are many options, but you will have to find out yourself, in person.
Good luck!