Market cap means nothing though. Facebook's size measured this way was bigger than all the publicly listed supermarkets in the UK combined, yet Facebook makes less profit than each of them individually.
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.)
Apple - 11.6 billion
Microsoft - 5.11 billion
Google - 2.89 billion
Facebook and Amazon both < 1 billion
Apple - $26bn Microsoft - $23bn Google - $9.7bn
So Apple edged past Microsoft in 2011. It is still some way from overtaking all of the companies combined.