The first internet-based film database which listed FBN was most likely OFDb. I'm not sure where they got the details from; most likely the official website or possibly from some early review.
After I was able to secure the New York showings for FBN I found myself listed on the IMDB and I also found a Metacritic and Rotten Tomatoes entry for the film, along with the requisite Amazon entry after Unearthed/Anthem announced the title for pre-order around February of this year. Then came the page at All Movie Guide, which I think many other databases extract their synopsis and cast/crew details from. For quite some time the only synopsis on Amazon was from the AMG but after I was able to connect the IMDB page with the Amazon DVD lisitng it automatically updated with itself with IMDB details and synopsis, though the non-North American variants of Amazon appear to be lisitng FBN in different ways - Amazon.ca still has the AMG details while Amazon UK
Then very recently I accidently found the DevilDead.com DVD entry, which was fun. The Encyklopedie hororových filmů lists FBN in it's database along with a bunch of stills; Horroweb.nl and MovieMeter.nl just have the poster designs. Also it appears that Blockbuster has a listing for the title though I'm unsure of whether they're going to order any actual copies. Somewhere along the lines it appears as if Yahoo Movies picked up on FBN as well. There's also these other websites such as Spout, Hollywood.com and Flixster which also have FBN pages.
One element which terribly complicates matters is the issue with the apostrophe in the title and the fact that I wanted it to be spelled without one but it's often added anyway. I don't really care either way (well I do but life's too short to rant and rave about this stuff) but the only problem is that if you search for "Frankensteins Bloody Nightmare" versus "Frankenstein's Bloody Nightmare" you will often get a whole different set of results. However, the important pages seem to list both title variants so I don't think it's a huge problem.