'Hey, are you still up? Am desperate for a fag!-will love you forever, please!;)' = reasonable;
'Hey sexy...have you gone to bed? Would love a fag and a bit of a chat, I'll behave myself...;) x = flirty, not ok, but not necessarily anything other than wanting a fag (I'd still hit the roof);
'Love you' 'Not tired yet' 'can I come down? Talk' = firstly, nothing to do with fags, so forget that excuse. Short, cryptic, very familiar messages. Very personal in their brevity. They've clearly had previous exchanges that have already set the scene, so to speak. What that scene is, you aren't going to find out. But it isn't innocent. You know that.
Have you ever seen that film The Tall Guy? The Emma Thompson character dumps the Jeff Goldblum character after a party, where she works out that he's been shagging someone else as she watches the off-the-cuff familiarity between them - passing drinks without the usual polite acknowledgement etc., the shaggers unconsciously act like husband and wife, taking each other for granted. And she clicks.
That's what those messages remind me of. ' can I come down? Talk' - that's how people in relationships message one another. And why does he need to specify that it's just to talk? (Um, he didn't think to specify that it was just to borrow a cigarette...) So what else might she assume her come down for?
Just be clear in your head about what you want, and please don't get bogged down tying yourself in knots trying to make yourself believe it's ok. It isn't. You're past that.
Good luck, I'm so sorry. And just remember that if you hadn't had the FB info, you'd be none the wiser tonight and they'd be seeing each other in work tomorrow.