I'm not saying you literally can't sell things in these countries, of course. I'm saying it has this massive barrier right now that creates a high threshold to entering and serving the market compared to the internet-enabled developed world, and that means less money for Facebook advertising in those areas at the moment than they could be having.
More advertisers means more demand means price-per-ad goes up and makes them more money.
More advertisers means more demand means price-per-ad goes up and makes them more money.