On stackoverflow, when a user has not written anything about him it says :
Apparently, this user prefers to keep an air of mystery about them.
Why in this case 'him' or 'her' isn't used instead of 'them'. In my mind, them is used when it's about multiple persons.
They can be used to refer to the singular as well as the multiple.
For example, The user left their profile empty.
Them is used instead of his or her when the gender is unknown or the speaker wishes to make a gender ambiguous sentence.
It is called a 'singular they'
