FB posting is too vague, plus you're assuming your friends will read between the lines and receive the hidden message.

That's way more complicated than speaking directly to them.

If they don't "get it" or figure someone else will take a pic since you haven't asked them directly, will you then start thinking they are ignoring your FB post?

Direct is the best way to go, beating around the bush usually involves alot of thorns.

SnapChat and FB Messenger both have cool graphics, why not just say you're curious to try it out...

That's usually how my daughter gets me to take a selfie, pleads cool graphics that I can't miss out on and deprive her from sharing with me.