> I wonder if a lot of these people who say 'X app is big in my country' actually just mean 'X app is popular within my social circle'.

Yes, that is exactly what it means unless there's some research data provided. Obviously it is easier to scale our current surroundings to the whole country, than to really go around and see how things are outside our social circle. Then again, same applies to you too, just because you haven't heard of WhatsApp before doesn't mean it wouldnt be big in the UK. :)