Facebook has now opened a new gender option, known as custom. With it you can define yourself as fluid or in whatever name you give to your gender. You may also pick your prefered pronoun that will be used by facebook when announcing your birthday and other events, so that people can use it about you.
Please repost this to get it as far and wide as you can, because this is something that is not widely known yet. Please reblog this and tag friends or family, anyone who you think wants or needs to know.