Why is the plural pronoun **them** used here?

Look at the following sentence that I just saw while viewing a profile:

Apparently, this user prefers to keep an air of mystery about them.

As it looks, them refers to this user, right?
This user refers to one person; however, the pronoun them is used which refers to many people and seems incompatible.

Can somebody explain why this sentence is right and why him/her aren't used?