Guys - asking him out on Facebook: how should I phrase it?

Anonymous
Met a guy at uni, we hit off pretty well but I couldn't tell if he was flirting or what he thought of me...I couldn't find the courage to ask for his number and he didn't ask for mine - uni is over now so unless I add him on Facebook and ask him out that way, I won't see him ever again. So how should I phrase it when asking him out? Start a casual private message conversation first or get straight to the point?

I thought I might do it like this:

-Start a casual conversation via Facebook private message and talk about general things like how exams went and plans for summer (its nearly summer in my part of the world)

-Finish the conversation with something like "anyway, reckon you'd like to catch up some time? if so...message me - (insert phone number here)...if not, have an awesome summer :)"

Is that too forward or too casual? I want him to know I'm interested but at the same time I don't want to be too forward.
Updates
+1 y
No, we don't have each others numbers and I won't be bumping into him in person as uni is over and I've finished...graduating - he still has a couple of years left of his course.
Guys - asking him out on Facebook: how should I phrase it?
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