Research: Google Ad Planner data on website user demographics could be wrong by up to 20%

A quick analysis we did today revealed that Facebook user figures reported in the Google Ad Planner platform can differ by up to 20% from the actual Facebook data.
Most online marketer probably know the Google Ad Planner platform and love the data it provides but might have wondered how accurate the user figures and demographics actually are for each website. Well, Google is not exactly open when it comes to revealing its data sources or methodology but it occurred to us that the Facebook ad targeting service would provide a perfect sanity check, at least when it comes to user data on
Summary of key findings
1. Total user numbers are very close
2. Google might have the genders wrong
3. User might not be as old as Google thinks
Although the Google data seems to be pretty accurate in terms of total user numbers by country when compared to actual Facebook data (numbers differ less than 0.05%), there seem to be significant differences when comparing user numbers by gender and age groups
The gender split between male and female users only seems to match in the US, whereas in Australia and the UK it is actually reversed, i.e. Google thinks there are more male users on Facebook than female users which according to Facebook data is exactly the other way around (user numbers differ by up to +/- 9% here). 
A similar pictures presents itself when looking at users by age group across Australia, the UK and US. Google seems to think that Facebook users are on average much older than they actually are across all three countries according to Facebook profile data (user numbers differ by up to +/- 19% here). 
Now don’t get us wrong, we love the Google Ad Planner and the data it provides but we hope that this simple analysis will get more marketers to actively question the data they’re looking at (and maybe even get Google to provide a little more transparency in the future).
You’re welcome to repeat the experiment by extracting the data yourself from the below sites or just download the raw data we collected as well as the Tableau workbook we used to analyse and visualise the data.
Facebook ad targeting service
Google Ad Planner platform (now DoubleClick)