I'm sure that Google and other large ad networks do this. But I also see small/medium size ecommerce retailers doing "People like you also bought" type stuff.

I'm sure that Google and other large ad networks do this. But I also see small/medium size ecommerce retailers doing "People like you also bought" type stuff. They don't do it as well as Amazon, but sometimes it seems relevant. So I'm curious about whether hundred of millions of impressions are necessary for this type of optimisation (and that the good recommendations I've seen are just a coincidence - quite possible) or if saying "hundreds of millions" actually just means "quite a lot"