A small gesture but I think it is a good start. If nothing else it may open up other peoples minds to realize that there are different ways to express our identities. Personally despite the fact that I am out to just about everyone I put "Female" on things like this as I don't see my being transgender as either adding or subtracting from identifying as female. I am not a different type of woman and I am certainly not expressing myself in way that I think differs from who I am... Again, personal preferences and I think this gives people the freedom to express themselves any way they want.
Way to go Facebook!
To update your gender go to your About section. Click "edit" and under the Gender dropdown select "Custom". You can then select what you would like to use as your gender. You can also select a pronoun to be associated with your profile.