Facebook is Alexa #5 in the US and has ~40,000 employees. Reddit is #6 and has ~500 employees. Twitter is #17 and has ~4,000 employees. (Numbers taken from quick Google searches)
They have some super talented engineers that work miracles with band aids and bubble gum.
Edit: Yes I understand not all employees work on everything. I was trying to give a sense of scale. Reddit is a tiny company with a massive website and no way to make money. If you have ideas - they are hiring.