I think using Facebook for marketing in general is problematic. Just because there are lots and lots of members does not mean that a good proportion are there to "find bargains" or "read about promotions" etc. i.e. most are there to, ermm, socialize.
So how to find that small subset of users that are interesting in marketing, specifically email marketing?
I don't know, but I imagine it's just a matter of putting yourself out there (~not~ "in their face"), and patiently waiting for them to find you.
Maybe the best thing to do would be to invest in some Facebook ads?