Lie.
i think working at facebook would be really fun. they use a lot of cutting edge technology and fun web technologies. no idea what the company culture is like though. if they had locations in places i would like to live i'd try to work for them.
in my 11.5 career as a dev, none of my jobs have been at software companies that i've actually used the software prior. i don't think you not using facebook would matter one bit.
If I was hiring, I would want someone that knows our product. I'd probably ask how would you improve our website, mobile app, etc. I'm surprised that they'd interview people that didn't use their product.
you can do all of that in about 15 minutes during the interview. just bring it up on the computer or phone and go over it.
and depending on what position it's for, his opinion on what could improve may not even matter. a good dev studio will have UX people who take care of improving usability, not have developers design it.
The questions would be to learn if they know about the product and if they have an opinion. Great way to learn how they think, interest and knowledge of the company and product. Hopefully, they ask a lot of questions from them.
Here's a list of some of the questions that they ask - 29 Questions That Were Asked at Facebook Job Interviews
During the interview, only answer there questions with emoji's.