I would look into uspto.gov to check and see if they have patent on fb. I am sure they do.

That's OK,there are tons of site like that

I'd say just go with it, I've heard of sites like that before, and to be honest I doubt there would be any legal ramifications, I think they would just shut you down.

you are the first one. there shouldnt be any problem with ir.

You can, but it can be disputed, too. More so if: a) fb really refers to Facebook,
and b) if Facebook is (likely) using the letters "fb" as a source identifier of their
service in commerce, which is what a trademark is whether it's registered or not.

(Should folks question if letters or acronyms can become trademarks, there are
examples like IBM, WB, or even KFC.)

Hint: type fb.com on your browser and see where that leads. Just a possible hint
anyway.

I'd say it should be okay - since you dont have the exact same keyword (facebook) in the domain, that shouldn't have any problem with the trademarks.

More so, there are bunch of fb related sites/ blogs/ out there on the net, so I'd say just go for it!

Thank you to all of you guys .I searched more and find that many of the domains which contain fb like are down.So i think its good to stay away and find some other alternative.

Even if you type the word fb in google , the first result is shown by google . com is facebook so yes fb is the brand name and i think it is better to stay away.

Thanks