Through constant improvement and outside the box thinking, google for instance has dominated the search engine market, taking out Microsoft, Altavista, Yahoo, AOL, Ask Jeeves etc, established companies all. Then went on to dominate the internet advertisement by using its own tracking resources and even more amazing thinking. Youtube did much the same thing, instead of downloading funny videos or email attachments, it was refined and polished streaming and turned it into a viable business and brought it to the masses. Hi5, WAYNE, Friendster, Myspace, Photobucket, Flikr and others have also been rendered largely obsolete under the Facebook umbrella. I get that some of these still have their uses Myspace for music, or say, Photobucket for hosting Avatars or questionable material.
Basically after the internet bubble burst, and companies were expected to run like businesses instead of hobbies few website devs have had such an impact as Google, Facebook and Youtube, and the continual improvements on their products have kept them relevant in a highly contested and fluctuating market. Tell me who did more for the internet than Google, and dont give me any of this legacy BS that doesnt mean squat now, or for connecting people, Facebook is the prime tool for internet communication, most people I know who are active on FB, which is to say, largely EVERYONE under 30 and many above that age use standard email outside of business use. Even MSN, AIM and Yahoo IMs are loosing tonnes of market share to FBs IM capabilities.
Even considering things like Ebay or Paypal, they made their statement and caused immense changes, but that are stagnating. They arent pressing forward with new ideas and improving on their model and expanding in any significant way. Unlike Google or Facebook which clearly have been.