Going to have to disagree entirely with CovertOperation here, and say that the facebook stuff doesn't bother me in the slightest.

Isn't the art of PU about taking a complete stranger and converting them to a close? Who's to say how and where that should be done. Like Midas said, I've got birds through all forms of locations; bars, clubs, public transport, facebook, PlentyofFish, Match, mutual friends, bookshops, Lush Cosmetics and even Primark. I don't care where the start is, whether online or offline if the result is the same.

End of the day, she agreed to go out with you, someone 16 years her senior, and then when her friends bantered you about the age, you crumbled.
There's two ways you can approach the age gap - positively or negatively. My flatmate is best part of 30 (as am I) and he's currently diddling an 18 year old, and the age banter happens all the time. Like I might say "we're going to blah blah blah today - do you want to come, or do you need your parents to sign a permission slip first?"

If you make jokes out of it all and show that you're unshakable, then they'll just go with it I find. If they don't and they take it personally, then they're probably not the kind of people you want to be going for anyway.

Seems to me that she's still interested (though blowing her off as she left may go down negatively), and if I were you I'd say something along the lines of "I had fun last night, though it would be nice to see you again without your bodyguards" (a line I've used a couple of times, and they always find the bodyguards bit amusing), and then go from there.

Keep us updated fella