Why stop at 51?

Recently Facebook updated the way that users could set their gender on the site:

Facebook has just updated to let users choose the gender pronoun they associate with. Aside from the usual “male” and “female” options, users can choose up to 10 different gender definitions to describe themselves out of more than 50 options, including “cisgender,” “transgender” and “intersex.”

Not only will this show up on the user’s About page, but it will show up in all other pronouns on the site that refer to that user. This way, users will not only show up as he/him and she/her, but some may show up with the neutral they/their.

Allow users to choose up to 10 identifiers from a list of 51 would provide users with roughly 16.5 billion different ways to specify their gender (assuming that Facebook isn't policing things and disallowing combinations like a cisgender male-female, which doesn't really work out).

Facebook updated their personal gender pronoun settings to allow users to choose from three different options, but their new options already appear to be more restrictive than those that some have already chosen to adopt for themselves. What about people who choose to identify as any of hum/per/thon/jem/ver/xem to draw just a few from Wikipedia's list of gender pronouns and why restrict it to only the possible pronouns that you might find such a location rather than allowing users to choose their own? The person referenced in this Slate article prefers the pronoun ou for some reason or other.

Why not allow users to go freeform rather than restricting them to just this somewhat larger selection of options for gender pronoun / identity? Three different personal gender pronoun options already fails to reflect the full spectrum of pronouns that users choose for themselves - will choosing a maximum of 10 of 51 gender identifiers eventually meet the same fate?