Definitely don't beat yourself up over it, every social media site evolves its own psychologically-optimum methods to hold our attention and keep us coming back.
This happens to be a golden opportunity for us to consider what exactly we want to get out of these services.
Sure, it's impossible to know with total accuracy what the true number of people is, but based on census data/birth and death rates/immigration and emigration stats, they've got a pretty reliable way of calculating an estimate.