I wonder which - programming language - platform - db type - host provider/ server type and configuration - that some big companies (Fb, twitter, groupon, foursquare...) used when they were started?
ex: Linux, Apache, Mysql, Php (LAMP) on Core2Duo 4GB ram, 2TB HDD; Python on Google AppEngine...
This would give me some idea on which one to choose for my startup :D
Well, you can google that information, but every company used something else.
For foursquare its MongoDB (NoSQL), with php, probably MySQL someplace there as well, and today they use Hadoop with Hive.
For twitter you can read here (Rails, MySQL)
For FB its LAMP to start with, and today its massive PHP/MySQL, Cassandra, Hadoop and Hive and probably many more, they are very open company about their engineering (they are proud at what they have done)
But if you are asking this to choose for your start-up the question is wrong.
You should plan you data and interactions between front-end and back-end, and decide which solution is best for you.
You need to also notice that today there are many cloud services - starting with one of them would make the most sense, you pay for what you use (close to nothing) and scale as required (that would be expensive, but if you get a lot of users that should not be a problem..)
You should not look at what a 10 million (and surly not a 800 million) user company does and try to do the same for 50 users - you will never get to a point where the 50 users will use what you are trying to build..
I would also start with something that I know, someplace where I will have an edge (for example, if you never touched .Net but you are very familiar with php, do not choose a C# based solution for your back-end - makes no sense)