[Mimu]

I would guess she searched for you.

[bro-stick]

Doubtful. Email and social graph correlation is usually how sites monetize to keep it "free" and survive.

[hoodoof]

But HOW? All she knows is my first name and suburb, and I have a very common first name.

[ProblemFactory]

With a more complex search perhaps?

Facebook parses and actually works with queries such as "single [gender] friends of friends of friends who live in [X] and like [Y]".

[notahacker]

If images used on the dating site also appear on Facebook then Google Images can do the rest...