'Dumb and Dumber To' -- Facebook Saved My Ass ... Says Billy the Blind Kid

Billy the Blind Kid from "Dumb and Dumber" better kiss Mark Zuckerberg's tuchus -- 'cause he'd have been screwed out of the sequel if it weren't for Facebook.

Billy -- real name Brady Bluhm -- was praying he'd get a call for part two once the studio confirmed it was a go ... but weeks passed without a peep.

Turns out the Farrelly Brothers had written his short, yet memorable part into the new script ... but didn't know if he was still in the business and couldn't find him.

Thankfully for Brady ... it's 2013 and they found him through FB. But business is business .. he still had to audition for the gig. He passed!

Brady says Harry and Lloyd still live in the same apartment for 'To' ... but beyond that he was tight lipped.