Free Comic Day has always confused me.
Is it to get new people into comics or readers into new issues they may not know they might enjoy?
It's never made much sense to me either, really, as far as getting new readers into comics. I have seen a few stores who set up at the theater where the latest Marvel blockbuster is running on FCBD and give out copies of comics, but other than that, I can guarantee that no non-comic reader wakes up on the first Saturday in May and says, "I hear they're giving away free comics at that greasy pit-stain of a store downtown. I'd better get down there!"
As a way to get current readers into new comics, however, it has worked. Last year I got hooked into Irredeemable because of the FCBD issue.