We analyzed some of the data Facebook makes available about the demographics of its users via its ad targeting system. We noticed that the number of male and female users did not add up: it turns out that some users do not indicate what their gender is - and that this tendency increases greatly as a user’s age increases. Below is a graph based on the US data indicating the percentage of users of each age (13 through 65+) indicated as male, female or neither.  While the “neither” number comes in at less than 10% of each of the largest Facebook demographic groups (18-22), after age 26 it increases fairly consistently - with 23% of 27 year-olds not providing a gender, rising to 40% of those age 36 and over 50% of all those 47 and older.