How should I be more direct through facebook?

[Jackie]

There's a younger guy (by 4 years) I met at a college party, and I randomly saw him the next day and we talked for about 10 minutes. He was really enthusiastic and really sweet (a little childish in an endearing way, but he's a football player so he's very tall and really hot and muscular). I was planning on sending him a facebook message (we are already friends on FB) saying:
' It was nice talking to you the other day. We should have lunch sometime! Let me know when you're free, here's my number'

Someone I asked about it said that asking him that way was:

" vague and meaningless. It's the chicken poo way of being forward-- you're asking him out but refusing to claim it by forcing him to name a time and follow up with you. If you're asking him out, be direct.
Don't forget- he's younger and a little childish so don't expect him to be the man."

How should I make it more direct? I don't have his number and he hardly checks his facebook, so this is the only way I can get his number because I hardly see him on campus. I'm just taking a risk; I only have 4 months left here, and we'll both be here over the summer so I want to trying dating someone my speed (I haven't dated before).

[Beagle]

Oh brother...what you said was fine! I say your original saying was perfect! If he's for it, he will follow up. If he's not, he will probably ignore it.