I've been straight with my kids from the start about things like Santa, the Easter Bunny, Tooth Fairy, etc. They are legends and fairy tales, but there is also something wonderful in those ideas and, because of this, we like play along with the legends and pretend they are real.
We still celebrate Christmas, exchange gifts, put up stockings, and the kids get presents from Santa on Christmas morning, but it is understood that Santa is just a legend that we pretend is real because, well... it's fun!
I've also explained that some parents don't let their kids in on the secret so a lot of kids think Santa is a real thing and just because you know the truth doesn't mean you need to ruin it for your friends. If their family treats Santa as a real thing just keep your knowledge to yourself and let them enjoy their holiday. One day they will learn the truth like you have.