> Market cap means nothing though. Well, it does. If it didn't, then the stock market would mean nothing. Market cap reflects the present value of future cash

> Market cap means nothing though. Well, it does. If it didn't, then the stock market would mean nothing. Market cap reflects the present value of future cashflows (basically future profits). Supermarkets tend to have very low profit margins, and there are physical and regulatory limits to their growth. The stock market is reflecting investors' prediction that Facebook's future profits will be bigger than several supermarket chains' combined. If you have reason to believe that is false, try and short $x worth of Facebook stock while buying $x worth of UK supermarkets stock. (Actually it's a bit more complicated than that in practice due to different currencies etc, so don't do it at home.)