They definitely used to have a startup culture, despite being big, not unlike Google now. A prolonged anti-trust dispute can wreck this because suddenly you get in big trouble for not doing things by the book.
By that token, you could also complain about the exclusion of IBM, Oracle, etc. These companies aren't really fighting it out for new territory the way Apple, Google, and Facebook are. They are big and relevant, but they're making money off existing fiefdoms.
Microsoft has the overall No.1 spot - it sells the OS and the software that virtually every office in the world (and most homes) rely upon. Nobody has come close to challenging that.
You can look at their other ventures - XBox, Bing, Zune, HD DVD, HotMail, SQL Server, IE, Windows Mobile/Phone and point out that they have not 'defeated' their rivals in these areas. But these are secondary or tertiary industries for Microsoft, where their main objective is to head-off any would-be challenges to their dominance of the PC market. Eg. consoles replacing the home PC, Google rendering it obsolete, Apple breaking into office sales.
 Yes Apple is the biggest company right now in financial terms, but Microsoft's turf is still the most desirable (though not quite as glamorous)
It is fascinating to see how much Microsoft used to be feared: "'Can you name anybody that's happy about being in the same industry with Microsoft?' Mitchell Kapor asks." -- http://www.around.com/microsoft.html
I like using the analogy of potential energy to think about this 'battlefield'. Each of the big 5 (or 6 if you include Samsung) has a large amount of potential energy they may be able to use over time to grow, entrnech, and profit. In the case of Apple, Google, MS and Samsung the PE is primarily profit.
In the case of Facebook, it is primsrily the social graph. For Amazon, the potential energy seems to be scale & efficiency.
I, personally, believe profit, and the ability to continue to generate profit is a better form of potential energy because it is more controllable.