One handy feature of Facebook ads is they allow you to target by users Interests. E.g.:
Now I haven’t played around enough with this targeting to determine how accurate or useful it really is, but one feature I like is that you can see the estimated number of Facebook users in Australia that are interested in a topic. For example when I have no Interests selected it displays:
When I select the category ‘Music -> Country’:
It displays:
So basically Facebook is telling you that out of the 9,021,820 people you can advertise to in Australia, 414,320 of them are interested in country music. I don’t know how accurate that really is but it is sure does provide you with another quick tool to work out the potential market size for your next product.