Remember when you had to talk about yourself as some sort of outside observer of your own thoughts on Facebook? In the early days, the status update frame was a fill-in-the-blank sentence (Arika is _______) that forced you to talk about yourself in the third person (…enjoying her breakfast). Later, under the influence of Twitter's more open-ended form, the frame dropped the "is" and used the prompt "What's on your mind?" At first people found the old third-person habit hard to break and persisted in using the username as the subject of every update (Arika ate an omelet for breakfast!), but eventually the username was moved out of the way and we started talking about ourselves in the first person (My breakfast was so good today. I love omelets!).
But the evolution of the status update didn't solve all of Facebook's third-person problems. Facebook didn't just report what you wrote, it would also talk about you to your friends. Before 2008, if you hadn't specified in your profile whether you were male or female, your friends might see updates like these: "Arika has changed their profile picture." "Arika commented on their status." If it didn't know whether to use his or her, it resorted to the old standby, singular they.
Singular they has a long, distinguished history, and though it is yet to be universally accepted as correct, it seems to be well on its way there. While sentences like "Everyone clapped their hands" and "Someone left their bag behind" don't ruffle too many feathers (they are at least less cumbersome than "Everyone clapped his or her hands"), sentences where the gender of the subject is known do sound a bit jarring with a singular they. People complained when they saw Facebook's singular they (especially with the word "themself" in updates like "Arika tagged themself in a photo"), and so Facebook came up with the solution of forcing people to chose male or female when creating profiles and asking those who hadn't specified before the requirement to make a pronoun choice:
People who wanted to keep the gender-neutral choice objected. Facebook explained that while they wished to respect groups "that find the male/female distinction too limiting" they also had to respond to "feedback from translators and users in other countries" where "translations wind up being too confusing when people have not specified a sex on their profiles."
Facebook's solution to the pronoun problem introduced a different sort of pronoun problem. People who didn't want to identify as male or female had to either switch to Google+ (which offered a choice between male, female, and other) or find a workaround. They eventually discovered some workarounds that involved changing elements of the Facebook code so that the old default with singular they showed up again, but one ingenious fix by Meitar Moskovitz goes even further than that, allowing the user to choose whatever pronouns "zie" wants. You can be zie, xe, thon, yo or anything you choose.
There have been attempts to introduce gender neutral pronouns since the 1850s. Dennis Baron, author of Grammar and Gender, keeps a list of them, ranging from hiser to zon, here. These were originally proposed for grammar reasons, to avoid singular they. Later, in the 1960s and '70s, they were put forward as tools for avoiding sexist language. Lately some of them have gained traction in transgender forums. But getting English speakers to embrace a totally new pronoun is a hard sell. English speakers already solved the problem long ago when they started using singular they (like Chaucer and Shakespeare did) which is already widespread in casual speech, if not in writing. Facebook is a mode of casual communication; they had the right idea in the beginning. Arika misses thons old profile.